Groups of youths on the edge of a major anti-austerity protest in central Athens threw rocks and firebombs at police outside Parliament, where the struggling government sought support for new cutbacks required to save the country from default.

At a rally of more than 20,000 in the Syntagma Square, police responded with tear gas to push the protesters away from barricades erected to protect the Parliament building and the lawmakers arriving to debate the new austerity plan.

Other demonstrators who had been part of the previously peaceful gathering also clashed with the violent groups of hooded youths, trying to eject them from their rally.

As the protests picked up, Prime Minister George Papandreou, who faces an internal party revolt from within his own governing Socialists over the austerity drive, was meeting with the country's president to discuss Greece's severe debt crisis.

The rally and two demonstrations which fed into it were part of a 24-hour general strike against the new cutbacks, which the country must pass in order to continue receiving funding from a €110 billion international bailout that is preventing it from defaulting on its debts.

A large part of central Athens was closed to all traffic and pedestrians as police mounted a huge security operation to allow lawmakers access to Parliament by car. Some 5,000 officers, including hundreds of riot and motorcycle police, used parked buses and crowd barriers to prevent protesters from encircling the building.

"Resign, resign," the crowd chanted outside Parliament.

The protesters included both young and old, and many brought their children, hoisting them onto their shoulders to shield them from the crush.

Such marches have often turned violent in the past, and three clerks died when rioters torched their bank during a mass demonstration in Athens last May.