Greek police clashed with protesters Wednesday as around 100,000 workers, pensioners and students marched to parliament in protest at austerity policies aimed at helping Greece cope with a huge debt crisis.

Riot police fired scores of rounds of teargas and flash bombs at protesters hurling petrol bombs, choking the main Syndagma square with smoke and sending crowds of striking protesters running for cover.

Public and private sector employees' 24-hour strike grounded flights, shut down schools and paralysed public transport in this year's first nationwide walkout against cost cuts.

In the biggest march since December 2008 riots brought the country to a standstill for weeks, about 100,000 Greeks marched through the streets of Athens chanting We are not paying and No sacrifice for plutocracy.

In several streets across the city, police fired teargas to disperse demonstrators hurling stones and plastic bottles. Shops barricaded their windows and hotels in central Athens locked up.

Police said two policemen and five civilians were injured, including one journalist slightly hurt by a petrol bomb. Four protesters were detained.

Protesters broke marble sidewalks for rocks to throw at police, set garbage cans on fire and damaged bus stops. Others unfolded a black banner reading We are dying in front of parliament.

We've reached our limits! We can't make ends meet, said 60-year old Yannis Tsourounakis, who has three children and is unemployed. Our future is a nightmare if we don't overturn these policies.

The Socialist government cut salaries and pensions and raised taxes last year despite repeated strikes, in return for a 110 billion euro (93 billion pound) bailout by the European Union and the International Monetary Fund that saved Greece from bankruptcy.

Greece's international lenders approved this month a fresh, 15-billion-euro tranche of the aid, but set a tougher target for privatisation proceeds and called for more structural reforms.

This strike kicks off a wave of protests this year with the participation of workers, pensioners and the unemployed. We are against these policies which are certainly leading to poverty and pushing the economy into a deep recession, Ilias Iliopoulos, general secretary of public sector union ADEDY, told Reuters.

Markets are watching for any derailment of Greece's fiscal efforts. Analysts say strikes are unlikely to make the government, which has a comfortable majority in parliament, change course but turnout in protests is a way to gauge its popularity.

The government has no room to change policies, said Costas Panagopoulos, head of ALCO pollsters. But most Greeks believe the burden is not equally shared and this is a problem.

Private sector union GSEE and its public sector sister ADEDY, which represent about 2.5 million workers or half the country's workforce, have vowed to resist austerity measures, saying they are killing the economy.

We can't take it anymore. I have been looking for work for many months while others are eating with golden spoons, said Thanos Lykourias, 27.