Rioters set fire to buildings and battled police in downtown Athens as the Greek Parliament approved Prime Minister Lucas Papademos's $160 billion austerity package to avert the nation's collapse.
Despite the protests, the Greek parliament passed the austerity bill needed for the bailout by the European Union and the International Monetary Fund.
As many as seven buildings - including a Starbucks cafe, a bank and a cinema, as well as other stores in downtown Athens - were set on fire, a fire department spokesman said, speaking on the condition of anonymity in line with official policy.
The buildings were near a bank that was set on fire in May 2010, killing three bank employees, during a general strike against Greece's first bailout package.
Greek police fired tear gas at petrol bomb-throwing protesters outside parliament, where tens of thousands had massed.
Police said some 80,000 protesters had gathered outside the building where debate on the plan imposed by the country's international creditors - the EU, the IMF and the European Central Bank - was ongoing before the late-night vote.
In the country's second city Thessaloniki, about 20,000 protesters took to the streets to protest against the austerity package they described as blackmail, which needs to be approved by parliament if Greece is to receive the bailout.
The unrest in Athens started when a group on Syntagma square tried to muscle past the police cordon protecting the parliament building.
Riot police retaliated with tear gas grenades, scattering protesters into nearby streets where they hurled rocks and molotov cocktails at the security forces.
People wearing masks smashed shop windows along two major avenues while a bank was set on fire, police said.
Sunday's protesters included trade unionists, youths with shaven heads waving Greek flags, communist activists and left-wing sympathisers, many of them equipped with gas masks.
Syntagma square was shrouded in a thick cloud of tear gas. One elderly Greek man could be seen among the demonstrators, breathing through a gas mask and wearing swimming goggles.
But while dispersing into nearby streets initially, the crowd soon returned onto the square, with families among the tens of thousands that had gathered.
A man was seen hawking paper masks - as some form of protection against the tear gas - as well as Greek flags.
Against the wall of the central bank, the word "Greece" was painted in black and replaced by "Bank of Berlin", alluding to the impression among Greeks that Germany is dictating the painful austerity measures.
"It's not easy to live in these conditions," said 49-year-old engineer Andreas Maragoudakis. "By 2020 we will be the Germans' slaves."
Another protester, Stella Maguina, 33, said: "We are here for our parents and our children, for all those who can't come."
Civil engineer Anastasia Papadaki, 27 said "the measures are not the solutions to the problem as they will not bring growth.
"It's just the international community blackmailing us."
"Enough is enough!" said 89-year-old Manolis Glezos, one of Greece's most famous leftists.
"They have no idea what an uprising by the Greek people means. And the Greek people, regardless of ideology, have risen."
Glezos is a national hero for sneaking up the Acropolis at night in 1941 and tearing down a Nazi flag from under the noses of the German occupiers, raising the morale of Athens residents.
"These measures of annihilation will not pass," Glezos said on Syntagma Square, visibly overcome by teargas and holding a mask over his mouth.
As is usual in Greek protests, only a small fraction of the crowd fought the police but one group started a fire right in front of a tent where first aid workers were preparing to care for the injured.
"Cops, pigs, murderers!" chanted the crowd.
Police said 14 injured protesters were taken to hospital - including one who was hit in the stomach by a flare - and at least 50 were treated at the scene for breathing problems caused by the tear gas. At least eight police were also injured.
Finance Minister Evangelos Venizelos, opening the debate in parliament, stressed the importance of backing the government-approved plan to stave off bankruptcy.
"The choice is not between sacrifice and no sacrifices at all, but between sacrifices and unimaginably harsher ones," he told a stormy debate expected to run well into the night.
One small party has already pulled out of the coalition of Prime Minister Lucas Papademos in protest against the terms of the rescue package from the European Union and International Monetary Fund - Greece's second since 2010.
A number of lawmakers from the two biggest government parties, socialist PASOK and conservative New Democracy, have also threatened to rebel but their numbers did not appear to be enough to sink the bill.
Greece needs the international funds before March 20 to meet debt repayments of 14.5 billion euros, or suffer a chaotic default which could shake the entire euro zone.
The EU and IMF say they have had enough of broken promises and that the funds will be released only with the clear commitment of Greek political leaders that they will implement the reforms whoever wins an election potentially in April.
Euro zone paymaster Germany ratcheted up the pressure yesterday.
"The promises from Greece aren't enough for us anymore," German Finance Minister Wolfgang Schaeuble said in an interview published yesterday in the Welt am Sonntag newspaper.
German opinion polls show a majority of Germans are willing to help, mr Schaeuble said, "but it's important to say that it cannot be a bottomless pit ... At least people are now starting to realise it won't work with a bottomless pit.
"Greece needs to do its own homework to become competitive - whether that happens in conjunction with a new rescue program or by another route that we actually don't want to take."
When asked if that other "route" meant Greece quitting the euro zone, Mr Schaeuble said: "That is all in the hands of the Greeks themselves. But even in the event [Greece leaves the euro zone], which almost no one assumes will happen, they will still remain part of Europe."
The austerity measures include cutting the minimum wage from about 750 euros a month and aim to cut Greece's bloated state sector workforce by about 150,000 people by 2015.
It also provides for a bond swap to ease Greece's debt burden by cutting the real value of private-sector investors' bond holdings by some 70 per cent. Greece will miss a February 17 deadline to offer a debt "haircut" to private bondholders if the vote is not passed today.