Police in Athens say about 25,000 people have gathered in the capital's Syntagma Square for a rally supporting a "no" vote in the upcoming referendum on whether to accept a new bailout deal.
Meanwhile, about 17,000 people gathered outside the nearby Panathenian stadium for the "yes" rally, police said.
Friday was the last day of campaigning before Sunday's referendum on whether Greece should accept creditors' demands for more austerity in return for bailout loans.
Brief clashes have broken out between a group of youths and police in the Greek capital's Syntagma Square just before the start of the main "No" rally backed by Prime Minister Alexis Tsipras' government.
Greek police used pepper spray Friday evening to deter several dozen anti-establishment protesters from throwing rocks and smashing property in the big Athens square in front of parliament.
The 'Yes' side was holding its rally just 800 metres away in a different section of central Athens. Friday was the last day of campaigning before Sunday's referendum on whether Greece should accept creditors' demands for more austerity in return for bailout loans.
The leader of Greece's conservative main opposition party says a "no" vote in Sunday's austerity referendum would drastically weaken the country's negotiating position with its creditors.
Antonis Samaras said Friday that all of Europe would perceive a "no" victory as a rejection of the euro currency itself — and that would result in Greek banks staying shuttered even longer, despite the government's vow they will open Tuesday.
Prime Minister Alexis Tsipras said earlier, however, that Sunday's referendum was not a vote on whether Greece will remain in the 19-nation eurozone. Tsipras backs keeping the euro currency.
Samaras also rejected Tsipras' claim that a deal with creditors could be hammered out within 48 hours after the referendum.
He called on Greeks to support a "yes" vote to keep Greece inside the eurozone.
In a rare move, 16 former armed forces leaders of Greece have signed a joint declaration calling on the Greek people to show "calm and national unity" ahead of Sunday's referendum on whether to accept creditors' demands for more austerity.
The letter said "Greece is at a highly critical moment in its history that will require difficult and inevitably painful decisions ... All Greeks, united and above party political lines and divisions, must help with all means available to address this situation with calm and national unity."
The letter was signed by three former chiefs of the armed forces, nine ex-army chiefs, two former heads of the navy and two former heads of the air force.
The Council of State, Greece's highest administrative court, has rejected an appeal by two citizens asking for Greece's critical referendum on austerity to be ruled unconstitutional. The vote will be held as planned on Sunday.
Court president Nikos Sakellariou said Friday "the referendum will proceed normally."
The reasoning behind the decision was expected to be issued later in the day.
The two men had appealed to the court on the grounds that Greece's constitution bars popular votes on fiscal issues. Greek voters are being asked whether to accept creditors' demands for more austerity in return for bailout loans.
A new opinion poll shows a dead heat in Greece's referendum campaign with just two days to go before Sunday's vote on whether Greeks should accept more austerity in return for bailout loans.
The ALCO surveyfor the newspaper Proto Thema gave the "Yes" campaign a 0.6 point lead over the "No" vote in the bailout referendum — well within the 3 per cent margin of error. Of those interviewed, 41.7 per cent said they would vote yes and 41.1 per cent intend to vote no, with 10.7 per cent undecided and the rest either casting blank ballots or abstaining.
The survey, released Friday just ahead of the final campaign rallies, also found that 76 per cent of Greeks want to stay among the 19 nations using the euro currency.
Asked whether holding the referendum was a good idea, 42 per cent said yes while 48 per cent said it was a mistake.
The poll was carried out July 1-3.
The eurozone's financial rescue fund — officially Greece's largest creditor — says it considers the country to be in default, even though Athens has not missed a repayment to the fund itself.
The European Financial Stability Fund said Friday it considered Greece's failure to cover a 1.6 billion-euro ($1.8 billion) repaymentthis week to the International Monetary Fund to be an "Event of Default" that allows the EFSF to activate a loan repayment demand. Fund CEO Klaus Regling says "this event of default is cause for deep concern. It breaks the commitment made by Greece to honour its financial obligations to all its creditors, and it opens the door to severe consequences for the Greek economy and the Greek people."
Top banking officials in Greece are meeting with Finance Minister Yanis Varoufakis, Deputy Prime Minister Yannis Dragasakis and Alternate Finance Minister Dimitris Mardas.
A finance ministry official says the meeting is being attended by the deputy governor of the Bank of Greece and the managing directors or presidents of the country's five largest lenders: Piraeus Bank, Alpha Bank, Eurobank, Attica Bank and National Bank of Greece as well as the secretary general of the Greek banking association.
The meeting comes as Greece gears up for its referendum Sunday and on the fifth day of restrictions on banking transactions, with Greeks limited to 60 euro daily withdrawal limits. Many ATMs have run out of 20 euro notes, meaning the limit for many has effectively been reduced to 50 euros.
The official, who spoke only on condition of anonymity in line with government rules, did not reveal what was being discussed.
Greek police say dozens of anti-establishment protesters have raided a supermarket in an Athens suburb and handed out food to people.
The incident happened in the Vyronas area, in the east of the capital, when about 30 protesters entered the supermarket, took food off the shelves and scattered pamphlets in and around the store.
Police say no one was hurt.
The head of the eurozone finance ministers' group, Dutchman Jeroen Dijsselbloem, harshly rejects comments by his Greek counterpart that Greece and its creditors have come "very, very close" on fiscal policy and reforms and that the only issue left is debt relief, which a "no" win in Sunday's referendum would lock into a deal.
Dijsselbloem says Yanis Varoufakis is totally wrong: "We are not talking to the government any more. They have chosen an extremely risky route, but we're now waiting for the result of the referendum."
"There are no new proposals from our side and, whatever happens, the future for Greece will be extremely tough. To get Greece back on track and the economy out of the slump, tough decisions will have to be taken and every politician that says that won't be the case following a 'no' vote is deceiving his population."
Germany's finance minister is dampening hopes that a quick bailout deal for Greece can be reached after Sunday's referendum.
Finance Minister Wolfgang Schaeuble was quoted as telling Germany's Bild daily on Friday that any negotiations after the Greek vote "will take a while." Greek voters are choosing Sunday whether to say yes or no to demands by creditors like Germany for more austerity.
Schaeuble said Greece would first have to apply for more aid and eurozone finance ministers would have to consider whether to open negotiations — which would have to be approved by the German Parliament. He insisted Greece would have to make efforts of its own in return for any new aid.
Schaeuble said: "These would be very difficult negotiations — because the situation in Greece has worsened dramatically in the past few weeks."
Greek Prime Minister Alexis Tsipras, in a televised address to the nation, has called on voters to reject creditors' proposals for more austerity in return for rescue loans.
He said Friday that Sunday's referendum is not a vote on whether Greece will remain in the euro. Tsipras backs Greek remaining in the 19-nation eurozone.
Tsipras also said an International Monetary Fund report released Thursday that says Greece needs debt relief "fully justifies" Athens' position that Greece's debt load is unsustainable. The Greek leader said Friday it was pointless to keep imposing harsh spending cuts on salaries and pensions just to continue servicing an unsustainable debt.
Speaking on the last day of campaigning before a referendum Sunday on the creditors' demands, the prime minister urged Greeks to vote "no to ultimatums, divisions and fear."
Greece' finance minister says an agreement with the country's creditors "is more or less done" as European officials have put forward "very decent proposals" to the Greek government his week.
Yanis Varoufakis has told Ireland's RTE radio Friday that this "has not been a dead week in terms of negotiations" despite European officials stating publicly that there would be no further talks until after Sunday's referendum.
He says Greece and its creditors "have come very, very close" on fiscal policy and reforms and that the only issue left is debt relief, which a "no" win in the referendum would lock into a deal.
Varoufakis says the ruling Syriza party "would emerge much stronger and united" if the "yes" vote prevails, playing down suggestions that the government would resign.
Luxembourg Prime Minister Xavier Bettel says he would have preferred that Greece take more time to organize Sunday's referendum and that the question on the ballot paper be far clearer.
Luxembourg has just taken over the rotating presidency of the European Union for the next six months and Bettel's government will have some responsibility for chaperoning through any Greek debt talks.
Bettel says that "the amount of time there has been to organize the referendum is not ideal. A referendum is something that requires an exchange."
He says that "a referendum is something where people have to know what they're voting on. People have to be able to understand the arguments in favour of a 'yes' or a 'no.'"
European Commission President Jean-Claude Juncker says a "no" vote in Sunday's referendum would weaken Greece's hand in its debt negotiations with international creditors.
Greeks will vote on whether to accept a proposal that creditors had made of specific reforms in exchange for loans. European Union institutions are framing it as a vote for or against the euro.
Juncker says Friday that "the Greek negotiating position would be dramatically weakened by a 'no' vote."
He adds that "even in the case of a 'yes' vote, we will have to face difficult negotiations."
Without sorely-need bailout funds, Greece could fall out of the 19-nation single currency club. However no legal mechanism currently exists for a country to leave the group or be kicked out by its partners.
The Council of State, Greece's highest administrative court, is hearing a case brought by two private citizens who are seeking to have Sunday's referendum ruled unconstitutional and cancelled.
A separate group has filed a counter-motion supporting the referendum's legality.
Spyridon Nicolaou is one of the two who are trying to have Sunday's popular vote called off.
He tells The Associated Press: "The referendum is invalid because it expressly violates the constitution, which stipulates that a referendum cannot take place on economic matters. But it's also invalid because it doesn't incorporate the text of the documents on which the Greek people are called on to decide. Would anyone from Evros (in Greece's far northeast) know the specific documents?"
Dimitris Belantis is one of those seeking to ensure the vote goes ahead.
He says the referendum doesn't violate the constitution, noting that votes can be held on "crucial national matters" which could include economic ones.
Greece's economic partners in the 19-country eurozone appear to be withstanding the escalation of the Greek debt crisis, with business activity rising to a four-year high in the second-quarter of the year, according to financial information company Markit.
Its purchasing managers' index — a broad gauge of business activity — rose to 54.4 in June from 53.6 the previous month. June's figure took the average index reading for the second quarter as a whole to a four-year high. Anything above 50 indicates an expansion in activity.
Chris Williamson, Markit's chief economist, says the turmoil in Greece appears to have had "little discernible impact on the real economy."
Williamson says the combination of the stimulus program from the European Central Bank stimulus and low inflation appears to be boosting consumer and business spending, helping to offset 'Grexit' anxiety.
Though the poll by ALCO for To Ethnos newspaper found Greeks evenly divided on the question that's actually being put forward in Sunday's referendum, it found a clear majority in favour of the country staying in the euro.
According to the poll, an overwhelming majority of 74 per cent said they believe it is better for Greece to remain in the euro, compared to 15 per cent who prefer a national currency. A further 11 per cent said they did not know or would not answer.
On the actual question about recent creditor proposals, the poll found 41.5 per cent supporting "yes" and 40.2 per cent "no," with 10.9 per cent undecided. The rest said they would abstain or leave their ballots blank.
ALCO interviewed 1,000 people nationwide on June 30-July 1 and has a margin of error of 3.1 per cent.
An opinion poll shows Greeks almost evenly split over Sunday's referendum on recent creditor proposals, with 41.5 per cent saying they will vote "yes" and 40.2 per cent "no," with 10.9 per cent undecided. The rest said they would abstain or leave their ballots blank. The difference between the "yes" and "no" votes is well within the margin of error.
Prime Minister Alexis Tsipras called the referendum last weekend, asking Greeks to decide whether they should accept creditor reform proposals in return for bailout funds. He is advocating a "no" vote.
The "yes" campaign says the referendum is a vote on Greece's future in the euro, which the government rejects.
The poll, conducted by ALCO for To Ethnos newspaper, interviewed 1,000 people nationwide on June 30-July 1 and has a margin of error of 3.1 per cent.
Germans are divided on whether Greece should stay in the eurozone.
An opinion poll conducted for public broadcaster ARD found that 45 per cent of Germans say Greece should keep the common currency. An equal number say Greece should leave the eurozone.
Sixty-nine per cent of respondents say they are concerned about the impact that a possible bankruptcy of Greece might have on the country's population. Some 31 per cent say they are worried about the cohesion of the European Union, while 24 per cent are concerned about the consequences that such a possibility might have for the German economy.
The telephone poll of 1,001 adults, published Friday, was conducted from June 29-30. It had a margin of error of up to 3.1 per cent.