Thousands of protesters are clashing with Greek police on the streets of Athens, hurling rocks and fire bombs at them on the first day of a general strike protesting a new round of national austerity reforms.

At least 4,000 police converged on the central part of the Greek capital Tuesday as about 20,000 people rallied in the square in front of parliament where lawmakers are considering Prime Minister George Papandreou's latest plan to raise taxes, cut government spending and sell state assets.

As the protests and the legislative debate consumed Greece, Christine Lagarde, the newly named head of the International Monetary Fund, called on opponents of Mr. Papandreou to support the austerity package, “in the spirit of national unity.”

Greece needs to adopt the plan by Thursday in order to avoid a default on loans it received last year as part of a $156 billion international bailout and to collect another $17 billion share of that assistance.

The protests initially were peaceful, with demonstrators marching through the capital banging drums and carrying banners attacking the terms of the bailout. Many of the protesters feel that the $40 billion austerity plan imposes harsh penalties on ordinary workers and pensioners while sparing the wealthy.

But the demonstration soon turned violent, with hooded youths ripping up paving stones and chunks of marble from building facades to throw at police. In return, police fired stun guns and tear gas at the protesters. Staff members at upscale hotels handed out surgical masks to tourists to avoid the tear gas, and helped them wheel luggage past the rioting.

Authorities said 21 police officers were injured in the rioting. But police made only a handful of arrests even as firebombs set buildings ablaze and tear gas filled the air.

The worker strike, planned for a second day on Wednesday, has all but shut down Greece.

Transportation, schools and other services were closed, as well as many private businesses. Hundreds of flights were canceled or rescheduled as air traffic controllers walked off the job. The air traffic controllers' union said airports would be closed for four hours in the morning and again in the evening, both Tuesday and Wednesday.

Doctors, casino workers, journalists and actors at a state-funded theater also joined the strike.

Greece faces a debt default as early as next month unless it secures the $17 billion from last year's bailout package from the IMF and EU. To qualify for the payment, the Greek parliament must pass the unpopular austerity measures.

European Union officials have warned that Greece has no choice but to adopt the austerity plan, contending a default would be calamitous for the 17 countries that use the common euro currency and the broader world economy.

Lawmakers began to debate the package Monday. Mr. Papandreou's plan includes five years of drastic spending cuts, the tax increases and the asset sales. He hopes to push the plan through the 300-member parliament using his socialist government's slim five-seat majority, but the outcome is uncertain.