Angered by high inflation, violent crime and high-profile corruption, and fearful President Cristina Fernandez will try to hold onto power indefinitely by ending constitutional term limits, protesters marched on the iconic obelisk in Buenos Aires chanting: "We're not afraid."
In a march organized on social media, demonstrators also converged on the pink presidential palace at the Plaza de Mayo square in scorching heat. There was little rowdiness, and the protest had the air of a family affair, with toddlers in strollers and grandparents in wheelchairs joining in.
People banged on pots, whistled and waved the Argentine flag. They held banners that read: "Stop the wave of Argentines killed by crime, enough with corruption and say no to the constitutional reform."
"I came to protest everything that I don't like about this government and I don't like a single thing starting with (the president's) arrogance," said Marta Morosini, a 74-year-old retiree.
"They're killing policemen like dogs, and the president doesn't even open her mouth. This government is just a bunch of hooligans and corrupters."
Police officials said the crowd numbered at least 30,000, while some local media put it at hundreds of thousands. People started heading home a little before midnight, although the Plaza de Mayo remained crowded.
Demonstrations were held on plazas across Argentina, including in major cities like Cordoba, Mendoza and La Plata. Protesters also turned up outside Argentine embassies consulates from Chile to Australia.
In Rome, about 50 protesters, all Argentine expats, held a noisy protest outside the consulate on Via Veneto. Among the slogans being shouted was "Cristina, go away."
About 200 demonstrators braved the rain in Madrid to bang pots outside the Argentine consulate.
"In Argentina, there's no separation of power and it cannot be considered a democracy," said Marcelo Gimenez, a 40-year-old from Buenos Aires who has been living in Spain for two years. "Cristina is not respecting the constitution. The presidency is not a blank check and she must govern for those who are for her and against her."
The protests hold deep symbolism for Argentines, who recall all too well the country's economic debacle of a decade ago. The "throw them all out" chants of that era's pot-banging marches forced presidents from office and left Argentina practically ungovernable until Fernandez's late husband, Nestor Kirchner, assumed the presidency in 2003.
"We came here because we don't want Cristina," said Shirley Brener, a 12-year-old student who protested in Buenos Aires next to her mother, Monica, a 48-year-old school director.
"Nobody can stand her," Shirley said.
The president's supporters sought to ignore two earlier protests this year, but when it became clear the latest effort could turn out huge numbers, her loyalists came out in full force. They dismissed the protesters as part of a wealthy elite, or beholden to discredited opposition parties, and misled by news coverage from media companies representing the country's most powerful economic interests.
In a speech Thursday, Fernandez didn't directly refer to the protest, but she defended policies that she said helped rescue Argentina from its worst economic crisis a decade ago and buoyed it during the 2009 world financial downturn.
"During boom times it's easy to run a country but try running when it's crumbling down," Fernandez said urging Argentines to continue to support her and pledging never to give up as her late husband taught her.
"Never let go, not even in the worst moments," she said. "Because it's in the worst moments when the true colours of a leader of a country comes out."
Buenos Aires Mayor Mauricio Macri, a member of the opposition and a fervent Fernandez critic, praised the big protest via Twitter. "People are being heard nationwide joined by a single flag," Macri said.
But polls suggest neither side has a firm grip on people's sympathies.
Fernandez easily won re-election just a year ago with 54 per cent of the vote over a divided opposition but saw her approval rating fall to 31 per cent in a nationwide survey in September by the firm Management & Fit. The survey of 2,259 people, which had an error margin of about two percentage points, also said 65 per cent of respondents disapproved of her opponents' performance.
Crime is the biggest concern for many of her critics.
Argentine newspapers and television programs provide a daily diet of stories about increasingly bold home invasion robberies, in which armed bands tie up families until victims hand over the cash that many Argentines have kept in their homes since the government froze savings accounts and devalued the currency in 2002. The vast majority of the crimes are never solved, while the death toll is rising.
Inflation also upsets many, as the government's much-criticized index puts inflation at about 10 per cent annually, or as little as a third of the estimates of private economists. As a result, real estate transactions have slowed to a standstill, given the difficulty of estimating the future value of contracts. And unions that won 25 per cent pay hikes only a few months ago are threatening to strike again unless the government comes up with more.
The phrase "Cristina or nothing" was stenciled onto the sides of buildings surrounding the Plaza de Mayo, the iconic square facing the presidential pink palace.
Demonstrators at Plaza de Mayo held signs accusing the president of arrogance. While some featured a lengthy list of demands. Others simply said "basta" — enough.
"I agree with many of the things she's done but I don't agree with her method," said Barbara Torino, a psychologist. "The state has to do everything within the confines of the law or else it can easily lead to state terrorism."
Associated Press writers Michael Warren, Almudena Calatrava and Emily Schmall in Buenos Aires; Frances D'Emilio in Rome; Jorge Sainz in Madrid; and Luis Andres Henao in Santiago, Chile, contributed to this report.