Tens of thousands of Czechs on Saturday staged one of the biggest protests since the fall of communism, marching in Prague against spending cuts, tax rises and corruption and calling for the end of a centre-right government already close to collapse.

Police estimated that 80,000-90,000 workers, students and pensioners snaked through the capital to rally in Wenceslas Square. Chanting and whistling, the crowd held banners proclaiming Away with the government and Stop thieves.

Rallies of such a scale are rare in the country of 10.5 million people.

The demonstration against Prime Minister Petr Necas's government is the third such trade union-led protest in 12 months against austerity measures, and the turnout underscored rising public frustration after a series of graft scandals.

This government is devastating state structures and is demeaning the unprotected with its asocial reforms, Jaroslav Zavadil, the head of the Confederation of Trade Unions, told the crowd.

The protest comes as the government is working to reaffirm its majority in parliament ahead of a Monday deadline.

The turmoil was triggered by the defection of Deputy Prime Minister Karolina Peake and her allies from the scandal-ridden junior ruling party Public Affairs.

Peake has pledged her faction will continue to support the cabinet, but on Saturday it remained uncertain whether she could muster the 10 votes the government needs for the safe majority that Necas wants from her to avoid early elections.

EARLY ELECTION?

An early election, two years after the last vote, would be likely to hand power to the opposition Social Democrats, who have a nearly 20 point poll lead over Necas's Civic Democrats.

The Social Democrats have pledged to undo some of the government's reforms of the pensions, healthcare and welfare sectors, and to tax companies and the rich to keep the budget under control.

The reforms are not thought-out. The reforms are chaotic, party leader Bohuslav Sobotka said before marching on Saturday.

It is essential that at this moment, Necas's government, which lost legitimacy with the breakup of Public Affairs, hand in its resignation and open the way to new elections.

Public Affairs has been riven by infighting and influential leader, Vit Barta, was given an 18-month suspended prison sentence this month for bribing party colleagues to stay loyal.

A March survey by the Public Opinion Research Centre (CVVM) found that people think official corruption is worst among political parties and in government ministries. The issue, along with the nature of reforms, is why many people took to the streets.

Corruption is quite bad and they are fighting it very little, said a protester, 30-year-old toolmaker Jaromir Tobias.

I agree with some of the reforms, but not with how they are explaining it and feeding it to the public. Reforms are necessary but not in this style.

The government survived another crisis earlier this month by agreeing to new hikes in sales and income tax as well as spending cuts worth 57 billion crowns ($3.02 billion) next year.

It says the measures are necessary to bring the deficit below 3 percent of gross domestic product in order to meet EU budget rules.

Unions said on Thursday the measures would cost the average wage earner 11,230 crowns a year - the gross average salary in the Czech Republic is 26,067 crowns ($1,400) a month.

With debate growing in Europe about how effective austerity measures are at reviving debt-choked economies, the Czechs are well-placed with a state debt load about half the European Union average, at 41.2 percent of annual economic output.

However, austerity and reform have hit domestic consumption, and unemployment hovers at around 8.9 percent. The $202 billion economy fell into a mild recession last year despite a record year for exports.

($1 = 18.8814 Czech crowns)

(Editing by Andrew Osborn)