By Jan Lopatka

PRAGUE (Reuters) - The leader of the Czech Republic's center-left Social Democrats said on Monday he would start talks with centrists on forming a government despite the fact that his own party ejected him from its negotiating team after a poor election result.

The Social Democrats won the largest share of votes in the weekend's parliamentary election, but its 20.5 percent score was the worst result since 1992 and a party leadership body has asked the leader, Bohuslav Sobotka, to resign.

Sobotka, 42, has defied what he called an "attempted execution" and told a rally of some 500-1,000 supporters he would push ahead with coalition negotiations with other parties.

"I am ready to start talks on formation of government," Sobotka said.

The party rebellion and Sobotka's refusal to resign has complicated the outlook for forming a government to replace a caretaker administration that has ruled the central European country of 10.5 million people since July.

Sobotka's move means his party will now in effect have two negotiating teams, the other led by the party's no. 2, Michal Hasek.

Sobotka called on his rivals to drop their actions and fall back in line to avoid leading parallel negotiations.

A quarter of a century after the fall of Communism in the 1989 "Velvet Revolution", Czechs have grown disillusioned with the political class and used their votes to protest against established parties, including the Social Democrats, that have been tainted by corruption scandals.

Financial markets have largely ignored the political turmoil that started with the collapse of a center-right government in June, but protracted paralysis could unsettle investors.

President Milos Zeman, who has long expressed a dislike for Sobotka, has in the past hinted he may prefer Hasek as prime minister. He told the weekly Tyden on Monday that if he were in Sobotka's place he would resign.

Sobotka's supporters carried banners at the rally calling on the party to reject "betrayal" by the Hasek faction and President Zeman.

"This was clearly a dirty trick," said Frantisek Dolezal, 67. "It was disunity from Hasek's side that caused the party to lose votes."

Sobotka said he would seek a wider party leadership body to decide his fate, hoping to overturn Sunday's vote by members of the party's elite.

The Social Democrats want to start talks with the centrist party ANO, an anti-corruption movement led by food and agricultural tycoon Andrej Babis, 59, that came second in the election. They also want to negotiate with the centrist Christian Democrats to form a three-party coalition.

ANO said its newly elected deputies and the party leadership would meet on Wednesday to decide the next course of action.

Social Democrat leaders have said they could agree with ANO on anti-corruption measures, such as laws requiring the publishing of public contracts. But they will struggle to follow through on plans to raise taxes for high earners and utilities and banks because of ANO's opposition to tax hikes.

(Editing by Robin Pomeroy)