Prague - Czech voters Friday began casting ballots in regional and Senate elections that could shake up Czech Prime Minister Mirek Topolanek's rule and affect the country's ratification of the EU Lisbon Treaty.

The Czech Republic has been divided into 14 regions since 2000. Their governments have been since dominated by Topolanek's senior ruling Civic Democratic Party.

Prague, also a Civic Democratic bastion, is officially a region but its government is elected in a municipal election that last took place in 2006.

Topolanek's party currently governs 12 out of 13 regions where elections are taking place.

Two years into Topolanek's rule, marked by unpopular belt-tightening measures, his party is expected to lose its dominance.

"The Social Democrats have understandably turned the election into a referendum on the government. Regional issues would not win them the election," said political scientist Tomas Lebeda.

Failure to defend "less than a half" of regions could cause Topolanek a serious headache, Lebeda said.

Leftist opposition and Topolanek's opponents inside his party, led by Prague's mayor Pavel Bem, could use a significant loss to unseat the premier.

The Social Democrat party already plans to call for a vote of no confidence next week, likely on Wednesday.

Czech voters are also picking 27 of country's 81 senators. Their choice could have an impact on the Czech Republic's ratification of EU's Lisbon Treaty aimed at overhauling bloc's institutions.

Topolanek's party currently holds 41 seats, a comfortable absolute majority that would enable mostly eurosceptic Civic Democratic senators to bloc the treaty. Such a veto could not be overruled.

Topolanek, who takes over as the bloc's president in January, signed the treaty, which is currently being considered by the country's Constitutional Court.

Unlike President Vaclav Klaus, the premier has spoken in its favour. But, Lebeda said, secret intentions could differ from official declarations.

The Civic Democrats are defending nine seats from the 27 that are up for grabs. Under such circumstances retaining the absolute majority should not be a problem, analysts say.

"The turnout will be low and that helps the Civic Democrats. They also tend to have strong candidates," Lebeda said.

Voting ends at 2 pm (1200 GMT) Saturday. The Senate election's second round in districts where no candidate wins an absolute majority this weekend is scheduled for next Friday and Saturday. (dpa)