The conservative-led coalition government's candidate for the presidential post, 73-year-old former European commissioner Stavros Dimas, garnered 168 votes from parliament's 300 seats — short of the 180 votes needed to win.
According to the country's constitution, parliament must now be dissolved within 10 days. Prime Minister Antonis Samaras said national elections will be held "at the soonest possible date" — Sunday, Jan. 25.
"The country has no time to waste," he said in a televised address just after the presidential vote. "I am here to guarantee that the country continues on a safe course ... so that the sacrifices made and the (economic) recovery are not endangered."
The government's fall — two and a half years into its four-year mandate — comes among deep uncertainty over the debt-ridden country's international bailout. Investors sold off Greek stocks Monday, with the Athens stock exchange's benchmark general index losing 7.3 per cent in midday trading, after falling as much as 11.3 per cent just after the vote.
Investors are worried that the main left-wing main opposition Syriza, which is consistently ahead in opinion polls, might try to renege on the terms of the bailout deal that is keeping the country afloat.
Syriza has pledged to roll back some of the reforms the country has implemented in order to qualify for billions of euros in rescue funds from other eurozone countries and the International Monetary Fund — although it has recently somewhat softened its rhetoric about unilaterally pulling out of the bailout deal.
"The main opposition party today forced elections," senior conservative lawmaker Dora Bakoyiannis said. "Elections at the worst possible moment for the country's economy."
Syriza leader Alexis Tsipras said Monday's vote marked a "historic day for Greek democracy."
"When the majority of the people is determined to end the policies of the bailout agreements and austerity, then lawmakers can do no else than respond to their duty to keep in line with the will of the people," he said.
"Today Mr Samaras' government, which for two and a half years plundered our society and had already decided and committed to take new measures, belongs to the past," Tsipras added. "With the will of our people, in a few days, the austerity agreements will also belong to the past.