Greece's two main labour unions called the strike to protest against the conservative government's efforts to reform Greece's debt-ridden and fractured pension system.

Unions claim the reforms will lead to lower pensions and higher retirement ages.

A police helicopter flew overhead as tens of thousands of strikers marched in bitter weather through central Athens, carrying banners reading "hands off our pension funds" and "the future belongs to the workers".

Shopkeepers along the protest route shut down their stores.

Unpopular reforms

A minor scuffle broke out when a small group of demonstrators threw rocks at riot police, authorities said.

Protesters take to the streets in Athens [EPA]

Police did not have a clear crowd estimate, but the demonstration was smaller than that held during the last general strike, in mid-December, when clashes between youths and riot police had caused several injuries.

More than 9,000 people also marched through the northern city of Thessaloniki.

Greece has about 170 separate pension funds, which collectively face estimated future deficits of between $165bn and $550bn, sums that are expected to affect the budget within a decade.

Costas Karamanlis, Greece's prime minister, has pledged to unify the funds as part of his reforms, but his efforts have proved unpopular.

The government has also been hit by a blackmail and sex scandal that has left it clinging to a single seat majority in the 300-member parliament.

The scandal looks set to hinder its ability to push through the reforms.