By Jason Hovet
PRAGUE (Reuters) - Tens of thousands of Czechs are expected to march on Saturday in what could be the country's largest protest against austerity measures, union leaders said on Thursday, as the centre-right government tries to fight off collapse midway through its term.
More than two dozen unions and activist groups will take part in the rally through Prague, the third major union protest in the past year against unpopular budget cuts and tax hikes that unions say are coming at the expense of workers.
The protest will be a test for the unpopular centre-right cabinet led by Prime Minister Petr Necas, which may lose its parliamentary majority and collapse due to a break-up of a junior coalition party.
Union leaders said turnout should top the estimated 40,000 that came out in May 2011, one of the biggest rallies since 1989 protests at Prague's Wenceslas Square that ended communist rule.
"I personally hope that Wenceslas Square will be full," said Jaroslav Zavadil, the head of the Confederation of Trade Unions.
Two years into its rule, the government's popularity is at new lows due to budget cuts that have hurt domestic consumption and growth but won praise from investors and ratings agencies.
The three-party government, which has gone through a series of graft scandals and rifts, was threatened again this week after the defection of Deputy Prime Minister Karolina Peake and her allies from the centrist Public Affairs party.
Peake has pledged her new faction will continue to support the cabinet. She has until Monday to secure enough votes to give the government a majority in parliament or face early elections.
Analysts mostly expect the cabinet to survive but Peake's faction was still short of the required strength on Thursday.
Zavadil said the latest political spat backed up the unions' demands for a new government. "It is playing like theatre against the Czech public and it is necessary to show clearly this government has no confidence," he said.
Unions said a new round of budget cuts that was agreed by the government this month will cost the average wage earner 11,230 crowns ($590) a year.
The government says hikes in sales and income taxes and spending cuts worth 57 billion crowns ($3.02 billion) next year are necessary bring the budget deficit below 3 percent of GDP.
The opposition Social Democrats, who have pledged to undo some of the reforms and tax companies and the rich to keep the budget under control, would handily defeat Necas's Civic Democrats, holding an almost 20 point advantage in polls.
Public Affairs has dropped below the 5 percent line needed to get parliamentary seats - showing not only growing discontent with austerity but also anger at suspected corruption.
Saturday's protests could be bolstered by the unions' cooperation with other citizens groups, including pensioners, disabled or students, that have also protested the government.
"What is quite clear is there is a strong mobilization of the public," political analyst Vladimira Dvorakova said. "Trade union protests are (traditionally) rather limited, but it seems it can have much stronger power now."
($1 = 18.9228 Czech crowns)
(Editing by Jan Lopatka and Maria Golovnina)