Greek protesters clashed with police on Wednesday as tens of thousands of strikers marched against austerity plans in a crucial test of the government's resolve in enacting deep budget cuts in return for EU/IMF aid.

Hundreds of demonstrators pelted police with rocks, chunks of marble and bottles, lit garbage cans on fire and tried repeatedly to storm parliament. But they were repelled by riot police hurling repeated rounds of tear gas that clouded up blocks of central Athens.

Masked youths threw petrol bombs, broke shop windows and shouted "Murderers" and "Burn the parliament," in a sign of swelling public anger at the government's plans for painful wage and pension cutbacks.

A giant plume of dark gray smoke rose over the central Stadiou Avenue, where a commercial building was burning. The fire brigade was evacuating people trapped on balconies. Three people died in the fire, the Greek fire brigade said.

"We have found three dead people in the building that is on fire," it said in a statement.

Police estimated the march at about 27,000 but eyewitnesses said there were at least 40,000, easily the biggest protest since Greece was first hit by a debt crisis late last year.

Public and private sector workers are staging their third joint strike this year. They have grounded flights, shut shops and brought public transport to a standstill.

"These measures are horrible," said Maria Tzivara, a 54-year-old saleswoman. "I'm afraid I'll get fired or my salary will be cut. It will be very tough."

Socialist Prime Minister George Papandreou submitted an austerity bill to parliament on Tuesday that envisages 30 billion euros ($40 billion) in new savings through deep cuts in wages and pensions and a rise in value-added tax (VAT).

The conservative opposition has vowed to vote against the bill, dooming hopes of a national political consensus on the measures.

The government enjoys a comfortable majority in parliament and expects to pass the legislation this week.

Despite polls indicating that most Greeks disagree with the austerity measures, protests had been fairly limited in past months and far more peaceful than riots that shook the country in December 2008 after a teenager was killed by police.

The strikes were unfolding against a backdrop of turbulence in financial markets.

The euro currency and stocks have sunk this week on concerns that the 110 billion euro ($142 billion) European Union/International Monetary Fund rescue package will be insufficient to stop the euro zone's debt crisis from spreading.

Concerns over the implementation of the "pain-for-aid" plan and contagion to other southern European countries are also raising questions about whether the 16-nation single currency zone can survive in its current form.

Opinion polls show increasing ire among ordinary Greeks, who feel they are being made to pay for their country's crisis while rampant tax evasion and corruption go unpunished.

The center-left daily Eleftherotypia warned on Wednesday that young Greeks could rise up if the government did not do something to give them hope.

"If nothing is done, the explosion of the angry and the disappointed will sweep away everything," the paper said "Have we forgotten December 2008 and become complacent?"

The president of the private sector union GSEE condemned the government measures as "harsh and unfair" and warned workers elsewhere in Europe that they could be next.

"Our struggle is also a message to the people of Europe that what started in Greece will soon spread because Europe has shown it is incapable of confronting this crisis," Yannis Panagopoulos told Reuters.