* Social Democrats almost sure to prevail but need allies
* May ask Communists for support, breaking taboo
* Uncertain outcome could rattle investors and markets
* Centre left plans to raise taxes for banks and big firms
By Jason Hovet
PRAGUE, Oct 25 (Reuters) - Czechs went to the polls on Friday in an election that will swing the nation leftwards after unpopular budget cuts and graft scandals felled a centre-right government.
The pro-European Union Social Democrats, promising to slap new taxes on big firms and high earners to pay for social programmes, were likely to win the most votes but fall short of a majority and find it hard to form a strong cabinet.
The party, out of power since 2006, may call on the Communists to support it in a minority government of the EU member state, breaking a taboo that has held since the 1989 Velvet Revolution that overthrew the Soviet-dominated ruling party.
Markets have mostly ignored the election thanks to the country's stability, underpinned by low debt load and the lowest borrowing costs in emerging Europe, but they may be rattled by an uncertain outcome and the risk of drawn-out coalition talks.
Polls opened at 1200 GMT on Friday and close at the same time on Saturday. Results are expected on Saturday afternoon.
"The right in this country is a catastrophe. (I'm for) European politics and social politics," said Vitezslav Zemanek, a 38-year-old translator who said he would vote for the Social Democrats or possibly the Green party.
With voters angry at established parties, especially on the right, a number of new groupings are likely to win seats in the 200-member lower house, which may force the Social Democrats to seek other, untested partners for a government.
These include the ANO ("Yes") party founded by Forbes-listed billionaire and food and agricultural magnate Andrej Babis, which shot up to the third spot in a final opinion poll on Monday, behind the Social Democrats and Communists.
"I will probably vote for ANO but not because I want to," said Vladimir Suchy, 57, a tour guide in Prague. "I won't give my vote to the crooks that have ruled up till now. It's a protest vote."
Social Democrat chief Bohuslav Sobotka aimed to win at least 30 percent of the vote to establish a strong mandate but could fall short because of the strength of new and smaller parties.
"We are ready to talk to parties on the left and centre," Social Democratic chief Bohuslav Sobotka told Czech Radio. "The more parties that make it to parliament, the tougher it will be to find a deal and the less stable the government will be."
The Social Democrats have pledged to keep the budget deficit below 3 percent of gross domestic product and will raise taxes on banks, utilities, telecoms companies and high earners.
They also plan to roll back unpopular pension and health reforms taken under the previous centre-right government of the Civic Democrats and conservative TOP09.
That administration unravelled in June, almost a year before scheduled elections, when a scandal over alleged illegal surveillance and bribery ensnared Prime Minister Petr Necas and his top aide, whom he later married.
TOP09 and the Civic Democrats are fourth and fifth in polls and have little chance of returning to any coalition.
But the Social Democrats' plan to ask the Communist Party for parliamentary support has not gone down well with many who recall its four decades of authoritarian rule.
Miroslav Kalousek, TOP09's deputy leader and finance minister in the previous government, said this election was the most important since 1989.
"It will decide whether we continue in our free democratic development or if we take another route," he said after casting his vote on Friday, according to CTK news agency.
Overnight on Friday, a group called Dekomunizace (De-communisation) put up a giant poster of Russian President Vladimir Putin in a Stalinesque pose, in a Prague park.
The protest was aimed both at the Communists and at President Milos Zeman, a Social Democrat whom critics have accused of trying to extend his powers since winning the country's first direct presidential election.