Demonstrators protesting Greece's latest austerity measures clashed with riot police Wednesday in Athens, as tens of thousands of workers massed on Greek streets for a two-day general strike.

The Athens protesters hurled rocks and fire bombs at police on Syntagma Square outside Parliament, with police countering with bursts of tear gas and stun grenades. A sentry post for the Tomb of the Unknown Soldier was set on fire, while some demonstrators smashed windows of nearby stores with chunks of marble they tore off buildings.

At least 100,000 protesters converged on the square, with many of them carrying signs calling for the ouster of the government. But the majority Socialist lawmakers gave preliminary approval to the austerity plan that would raise taxes and cut government jobs, with a final vote set for Thursday.

Another 50,000 demonstrators took to the streets in Thessaloniki, Patras and Heraklion, part of a nationwide work stoppage that has left massive piles of uncollected garbage on Athens streets, closed popular archaeological tourist sites and shut government operations. Most of the country's professionals — including doctors and teachers — joined the 48-hour strike, along with bakers, taxi owners and gas station operators.

While the Greek protests unfolded, key European leaders — including French President Nicolas Sarkozy and German Chancellor Angela Merkel — headed to Frankfurt to discuss ways to resolve the continent's debt crisis. It was a preliminary meeting ahead of Sunday's scheduled European Union summit in Brussels.

The EU leaders are considering a broad plan to boost the continent's bailout fund for debt-ridden governments, as well as stabilizing the continent's banks even as they are forced to assume bigger losses on the Greek loans they hold.

Greece's international creditors have demanded that the Athens government approve the austerity plan as a condition of securing another segment of its $159 billion bailout from last year so it can avoid a default on its loans next month.

Greek Finance Minister Evangelos Venizelos told Parliament that it “must explain to the citizens” that the austerity measures are “absolutely necessary” so the country can avoid an even worse economic fate. Prime Minister George Papandreou has implored Parliament to pass the new austerity plan. Opponents of the Socialist government said its policies have been a failure.

Previous spending cuts have failed to make a dent in Greece's massive debt, which stands at 162 percent of its economic output.

Greece's economy is mired in the third year of a recession. The country's jobless rate hit 16.5 percent in July, just below the record set in May. Young workers were the hardest hit, with two in five without a job.

Unions in another financially troubled country, Portugal, have called for a nationwide general strike on November 24. They are upset that the government plans more spending cuts, including on salaries for government workers.

Portuguese Prime Minister Pedro Passos Coelho has called the country's financial state a national emergency. But Portugal was required to adopt the spending cuts and tax increases so it could secure its $108 billion international bailout earlier this year.