Riot police fired tear gas while masked youths hurled firebombs at authorities, as thousands marched in Athens [AFP]

Clashes broke out in Athens, the Greek capital, amid large protests against austerity measures aimed at curbing the country's massive debt crisis.

Riot police fired tear gas and flash bombs at protesters in Syndagma square on Wednesday, while masked youths hurled firebombs and threw rocks at authorities, in clashes that lasted for three hours.

Police said five civilians and two officers were injured,  and another 20 were detained, while a policeman was attacked and his uniform set on fire, before he was rescued by colleagues.

The year's first nationwide walkout by public and private sector employees against spending cuts saw flights grounded, schools shut down and public transport paralysed.

Up to 100,000 people marched through the streets of Athens shouting "We are not paying" and "No sacrifice for plutocracy" before violence broke out.

Greece's Socialist government cut salaries and pensions and raised taxes last year despite repeated strikes, in return for a $150bn bailout by the European Union and the International Monetary Fund.

"This strike kicks off a wave of protests this year with the participation of workers, pensioners and the unemployed," Ilias Iliopoulos, general secretary of public sector union ADEDY, told the Reuters news agency.

"We are against these policies which are certainly leading to poverty and pushing the economy into a deep recession," he said.