Youths broke up marble paving slabs in central Syntagma Square and hurled the chunks at police in full riot gear. The police responded by firing teargas grenades and chasing the protesters through the square and into surrounding streets.

Flights were grounded, schools shut and government offices closed in Greece's first nationwide walkout in months. Labour leaders call it the start of a campaign to derail emergency austerity steps launched last month by a government that has already imposed two years of tax hikes and wage cuts.

Greece's worsening debt crisis poses a risk to the euro currency and the international financial system. Reforms to Greek finances took on a new urgency this week after the government announced it would miss its 2011 deficit target.

Thousands of state workers, pensioners and students had gathered peacefully, beating drums and waving banners reading "Erase the debt!" and "The rich must pay." They marched into the square outside parliament where lawmakers were debating holding a referendum on the response to the fiscal crisis.

Violence was far milder than in June, when more than 100 people were injured in battles between demonstrators and police in Syntagma Square.

Despite its new measures demanded by the EU and IMF, the government was forced to announce this week it would still fall short of its 2011 deficit target by nearly 2 billion euros, rattling globalmarkets. Polls show nearly four of five Greeks expect a default on the massive national debt within months.