PRAGUE (AFP) -- Czech Prime Minister Mirek Topolanek's party suffered heavy losses to the opposition Social Democrats (CSSD) in regional elections, official results showed on Saturday.

The result led Social Democrats leader Jiri Paroubek to call on Topolanek to resign before Prague takes over the European Union's rotating presidency at the start of 2009.
""If I were in (his) position, I would leave of my own accord,"" Paroubek told Czech television, adding that Topolanek at the head of the EU for six months was an ""unacceptable"" situation for his party.
Instead, Paroubek called for a ""cabinet of experts"" to run Prague's six months at the EU helm.
""Voters opted for change,"" he added after what he said was a ""huge victory.""
Paroubek's party is planning to introduce a censure motion against Topolanek's government on Wednesday. ""We'll see how that vote goes,"" said Paroubek, prime minister in 2005 and 2006.
In parallel senatorial elections, the social democrats are also on course to make significant gains going into a second round of voting -- with potential implications for Prague policy on the European Union's Lisbon Treaty and the sitting of a U.S. anti-missile radar site.
The main opposition had pitched the vote as a mid-term referendum on Topolanek's center-right government, and claimed 280 of 675 regional councilors seats, spread evenly across 13 voting regions.
In the senatorial poll, Social Democrat candidate Radek Susil won 53.34 percent of the vote in the north-eastern Karvina district, meaning he was elected without the need for a second round.
The party has made it through to the second round in 25 of the 26 constituencies, whereas Topolanek's governing Civic Democrat Party (ODS) will only be on the ballot papers in 20.
A third of the upper house's 81 seats are up for re-election, with the second round to be held October 24-25.
Topolanek has come under fire in recent weeks from opposition politicians and voters after agreeing on a deal that will allow the United States to base an anti-missile radar system on Czech soil.
Less than three months from the country's taking up the baton of the European Union presidency from France, Prague's position on the EU treaty has also come under close scrutiny.
The regional elections are considered an important political test for the fragile ruling coalition of ODS, Christian Democrats and Greens, which does not have a clear majority in the parliament's lower house.
Voter turnout was estimated at around 40 percent.