Athens - Greek riot police fired teargas against hooded protesters during an anti-austerity march on Thursday, one day after a national unity government took office charged with imposing painful tax rises and spending cuts to avert bankruptcy.

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 honour 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 towards an absolute majority to implement our programme 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 totalling 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.” - Reuters