Tens of thousands of demonstrators jammed central Athens and the northern city of Thessaloniki on Wednesday as a general strike to protest government plans to reform the country's debt-ridden pension system brought Greece to a standstill.

A police helicopter flew overhead as tens of thousands of people — with some estimates putting the number at 80,000 — marched through central Athens in two separate demonstrations. In Thessaloniki, 30,000 people marched through the center, police said.

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Clashes broke out in one central Athens square, with riot police firing tear gas and charging a group of about 300-400 black-clad youths minutes after a car parked nearby was smashed and set on fire. No injuries were immediately reported.

The strike shut down hospitals, banks, schools, courts and all public services. Archaeological sites, including the Acropolis, were closed, and even some telephone directory inquiry lines, motorcycle couriers and pizza delivery men walked off the job.

Some private-run banks were open, but public transport, including boats connecting mainland Greece with the islands, ground to a halt. The capital's metro and tram systems were operating for about five hours to allow strikers to join two rallies and marches in the center of Athens.

Planes were grounded as air traffic controllers joined the 24-hour strike, which began at midnight Tuesday. Hospitals were accepting only emergency cases. Courts were shut for a second day after lawyers and judges began a 48-hour strike on Tuesday.

Journalists also walked off the job, canceling television and radio news programs. The three state-run television stations were broadcasting only a text stating their support for the strike. No Thursday newspapers were to be published.

Bystanders applauded uniformed Olympic Airlines pilots and flight attendants who joined the march. The transport minister said recently that the debt-ridden flag carrier was unlikely to survive in its current form. A decision on its future is expected next year after meetings with EU officials about hundreds of millions of euros of illegal state aid to the airline.

"We are ... fighting for the savior of the national air carrier of Greece, fighting along (with) the other employees" against the government's planned reforms, said airline employee Dimitris Papadimitriou.

Giorgos Skiadotis, who handles union issues for Greece's main communist party, the KKE, said that "workers should not pay for the mistakes and mismanagement of past governments in handling the pension funds."

Skiadotis accused the right-wing government of attacking the country's social security system. "We must all stand together ... to defeat this attack," he said as he joined the demonstration.

More strikes are expected next week, with medical professionals, including pharmacists and dentists, walking off the job for 24 hours on Wednesday and courts shutting down on Wednesday and Thursday.

Labor unions oppose government efforts to reform Greece's fractured pension system, including attempts to unify the country's roughly 170 pension funds. Many fear the reforms could cut pensions and raise retirement ages.

The funds face estimated future deficits collectively of between euro120 billion and euro400 billion (US$165 billion and US$550 billion) — sums that are expected to affect the budget within a decade.