Riot police take up positions behind flames from a petrol bomb during clashes with protesters in Athens on Thursday. (John Kolesidis/Reuters)

Protesters clashed with riot police as tens of thousands of people marched through central Athens on Thursday during a nationwide general strike against extensive spending cuts by the country's socialist government.

Hundreds of masked and hooded youths punched and kicked motorcycle police, knocking several off their bikes, as riot police responded with volleys of tear gas and stun grenades.

The violence spread after the end of the march to a nearby square, where police faced off with stone-throwing anarchists and suffocating clouds of tear gas sent patrons scurrying from open-air cafes.

Protest organizers said 60,000 people participated in the march, while police said 20,000 people were involved.

Police say 16 suspected rioters were detained and two officers were injured.

Minor clashes also broke out in the northern city of Thessaloniki, where about 14,000 people took to the streets.

Airports, offices closed

Thursday's strike closed airports, public-sector offices, hospitals and schools throughout Greece as labour workers protested the harsh austerity measures. The government announced last week it would use the measures to trim the country's ballooning deficit and shore up the support of skeptical markets.

Public- and private-sector workers march during a 24-hour labour strike in Athens on Thursday to protest austerity plans. (Yiorgos Karahalis/Reuters)

It was the third such strike in just over a month.

Even police officers, firefighters and coast guard officials participated, some 200 marching in uniform in Athens to express their discontent.

"The police and other security forces have been particularly hard hit by the new measures because our salaries are very low," said Yiannis Fanariotis, head of a police association. He said the average police officer makes as little as $1,400 Cdn a month.

Greece is under intense pressure from the European Union to improve its finances quickly. It announced two weeks ago that it would cut public-sector salaries, freeze hiring and pensions and hike consumer taxes to generate $65.33 billion in savings.

The cutbacks, added to a previous $15.24-billion austerity plan announced in early February, seek to reduce the budget deficit from 12.7 per cent of annual output to 8.7 per cent this year.

That prompted workers to launch a first general strike on Feb. 10. A second strike happened on Feb. 24.

The long-term target is to bring overspending below the EU ceiling of three per cent of GDP in 2012.

Unions cry foul

The government says the tough cuts are its only way to dig Greece out of a crisis that has hammered the common European currency and alarmed international markets — grossly inflating the loan-dependent country's borrowing costs.

But unions say ordinary Greeks are being asked to pay a disproportionate price for past fiscal mismanagement.

"They are trying to make workers pay the price for this crisis," said Yiannis Panagopoulos, leader of Greece's largest union, the GSEE.

"These measures will not be effective and will throw the economy into deep freeze."

Thursday's strike also shut down ferry service and halted news broadcasts. Journalists, teachers, state hospital doctors and air traffic controllers are among those striking, while officers from the police, fire service and coast guard plan to join protest rallies scheduled for later Thursday.

With files from The Associated Press