At least 50,000 people marched past shuttered shops in central Athens beating drums, waving red flags and chanting "EU, IMF out!" in the first public test for technocrat Prime Minister Lucas Papademos' fractious three-party coalition.
The annual November 17 march commemorates a bloody student uprising against Greece's military junta in 1973 but often becomes a focal point for anti-government protesters.
Unions have said they would use this year's rally to send a warning to Papademos, a former vice-president of the European Central Bank with no political experience, to reverse policies they say have sent Greece into a "death spiral."
"They have cut my pension twice. This man Papademos is worse than the previous leader. He is a banker. If he dares to take any more austerity measures, we will throw them out," said pensioner Xeni Kolen, 64.
Youths hurled stones and petrol bombs at baton-wielding police. Schools, universities and many businesses stayed shut for the day and public transport was badly disrupted. A smaller protest rally took place in Thessaloniki, Greece's second city.
Police detained dozens of protesters in Athens. Despite the clashes, police said the rally had passed relatively peacefully and protesters later dispersed without major incident.
Three in four Greeks back Papademos, opinion polls say. But he faces a Herculean task keeping his coalition united behind reforms required under a 130-billion-euro bailout aimed at preventing a default.
"Greek people and above all the young can overcome the crisis and achieve national targets if they are united and act decisively," Papademos told parliament earlier on Thursday.
He evoked the memory of the 1973 uprising against the colonels who ruled Greece in 1967-74. Its suppression hastened the collapse of their dictatorship. Parliament observed a minute of silence to honor the dozens killed in the uprising.
Cracks are already emerging in Papademos's coalition, cobbled together after Greece's euro zone creditors rejected a plan last week by then-premier George Papandreou to hold a national referendum on the bailout.
The leader of the New Democracy conservatives, Antonis Samaras, whose party is ahead of rivals in opinion polls, made clear his support for the coalition was only temporary and that he was preparing for an election slated for February 19.
Samaras has infuriated Greece's EU peers over the past two years by opposing measures pursued by Papandreou's Socialist administration to stave off default.
In an interview for the magazine 'Epikera' on Thursday, Samaras said his party would not cooperate with its rivals beyond the election and reiterated his call for a switch to pro-growth policies to end a four-year-old recession.
"We are working toward an absolute majority to implement our program without delay," he said. "When we can, we will change all that needs to be changed. But to do that, we will need a strong mandate in the coming election."
Many Greeks blame the two main parties, dominant since the fall of the junta, for piling up debts totaling 370 billion euros, or 160 percent of gross national product.
But Papademos's government, strongly backed by the European Union, is vulnerable to charges that it lacks legitimacy because it has not been chosen by the voters.
"I believe we must go to elections... This government is unconstitutional. We have not voted for it," said Vassilis Papadopoulos, 49, a bank employee who joined Thursday's rally.
"It does not represent us."
Samaras repeated that he would not sign a written pledge demanded by Brussels swearing to do what it takes to see through the 130 billion euro bailout. He says his word is enough.
His stance has irked Greece's EU partners who fear Athens may try to wriggle out of its obligations. They have started to speculate publicly about whether the Mediterranean country of 11 million has a future in the euro zone.
Greece's two-year debt saga has morphed into a major crisis threatening the very existence of the euro as contagion drives up the cost of borrowing in much bigger economies such as Spain and Italy and now even France.
Mirroring events in Greece, a new Italian government led by technocrat Mario Monti unveiled sweeping reforms on Thursday.
The International Monetary Fund, Greece's main creditor along with the EU, said on Thursday it too wanted assurances from the new government on its commitment to meeting economic targets before it would agree to release the next aid tranche.
"We stand ready to work with the coalition and once broad political support (for) measures under Greece's economic program is assured, then we can proceed with completion of the fifth review," IMF spokesman David Hawley told reporters.
In addition to the new 130-billion-euro package, Greece needs 8 billion euros from a previous bailout to meet debt maturing next month. Without the cash, it will default.
Representatives of the EU, the IMF and ECB -- the so-called 'troika' -- will start arriving in Athens on Friday for talks on releasing the next aid tranche, a troika source said.
Also on Friday, the government will submit to parliament a 2012 budget of more tax rises and spending cuts.
The European Commission's task force for Greece said tax avoidance and lack of compliance had cost the country some 60 billion euros, equivalent to about 25 percent of Greek GDP. [ID:nL5E7MH1RX]
Under another key part of the bailout plan, Greece has begun talks withprivate sector bondholders on a bond swap which aims to halve the debt Greece owes to them.
"Our goal is to structure a transaction that will attract the broadest possible support from the bondholder community," said Evangelos Venizelos, the minister of finance.
The ministry later said it expected to present a proposal to the private bondholders by the end of November.
In Frankfurt, Charles Dallara, head of the Institute of International Finance representing the banks, said the 50 percent 'haircut' for private holders of Greek debt should help restore market confidence. He ruled out a bigger hit for creditors.
($1 = 0.739 Euros)