Riot police fired tear gas at demonstrators throwing rocks and firebombs in central Athens during a nationwide general strike Wednesday by millions of Greeks protesting government pension reforms.

An estimated 100,000 people marched in downtown Athens, and when the demonstration ended, groups of anarchists fought running battles with riot police in the capital. Clouds of tear gas hung over Exarhia Square and cafe customers scrambled for cover.

Clashes had broken out during the march when a group of demonstrators threw two or three fire bombs and rocks at riot police outside Parliament, and police responded with tear gas. About 8,000 people also marched in the northern city of Thessaloniki, where protesters set fire to two banks and three automatic teller machines. No injuries were immediately reported in either Athens or Thessaloniki.

Wednesday's walkout, like two other general strikes since December, shut down public services and forced the cancellation of dozens of flights.

"We're marching for a socially just pension system," Socialist opposition leader and former Foreign Minister George Papandreou told AP Television News earlier as he marched through downtown Athens.

He accused the government of eroding "the most basic of pension rights," particularly for women, while offering tax cuts for the rich and benefits for large corporations.

"It's unacceptable. We're fighting and we hope we can win this fight for a much better and much safer Greece," he said.

Opinion polls show most Greeks oppose reforms to the fractured and debt-ridden pension system that would unify pension and health funds, raise the effective retirement age for women and working mothers, and create both incentives and disincentives to keep employees working longer.

"No to the continued robbing of our pension funds and rights," read a banner carried by protesters in Athens. "We will not back down," read another.

Hospital doctors, air traffic controllers, teachers, port workers, hotel employees and gas station workers joined others already striking this week over the fiercely contested reforms.

Thousands of Greek workers walked out Tuesday when a 24-hour strike by rail workers also brought subway and train transport in Athens to a standstill. Banks shut down and most courts were empty because of a weeklong lawyers' strike.

Mounds of trash continued to pile up on city streets Wednesday because of a garbage collectors' strike, while strikes by employees at the main power company have caused rolling blackouts for the past two weeks.

Since winning re-election in September, the government has pushed to change the pension system, warning that it could collapse in a few years' time if it is not reformed.

Yiannis Panagopoulos, head of one of the two main labor unions, GSEE, said unions would continue to resist the changes, even if they are voted into law Thursday.

"The battle doesn't stop with the vote on the legislation," he said Tuesday. "The legislation needs to be implemented when it becomes law. And there resistance will reach its climax."