By Robert Muller

PRAGUE (Reuters) - The Czech interior minister on Wednesday signed a contested plan merging top anti-corruption units in the police that is at the center of a row shaking the center-left government.

The plan to reshuffle police top structures, bringing together organized and economic crime units, is testing the three-party coalition after more than two years of relatively smooth rule.

Finance Minister Andrej Babis's ANO party has rejected the plan and said it would seek changes to the coalition agreement after the reform was signed over its objections.

Babis, who had first threatened to quit the coalition last week after the police reforms were suddenly announced, has called for more debate on the plan, saying its approval violated the government's coalition agreement.

"We need to make a restart and to say whether we will begin anew," Babis said after a meeting of the ANO leadership.

Milan Chovanec, the interior minister and a member of the prime minister's Social Democrat party, denied the reforms, due to take effect Aug. 1, violated coalition rules when he spoke at a news conference on Wednesday.

He said he had consulted Prime Minister Bohuslav Sobotka, who left earlier in the day for a planned trip to China to last until June 21, before signing the plan.

"I resolutely reject politicizing the police," Chovanec said. "We are prepared for a factual debate... We reject this political game with the police."

Babis said he would wait for Sobotka's return to negotiate a new coalition deal, saying repeated coalition agreement violations made it difficult to continue in the grouping.

The police reshuffle has surprised ANO and state prosecutors worried over the impact it will have on open cases and risks of evidence leaking. The head of the organized crime unit has resigned in protest.

Czechs have grown used to political instability, with cabinets since 2002 either switching prime ministers or collapsing during their term.

But, thanks to low debt and strong institutions, the European Union member maintains the best credit rating and lowest bond yields among central European neighbors.

The Social Democrats and ANO are the two most popular political parties and will contest regional elections this fall. The Christian Democrats are a junior partner in the coalition.

(Reporting by Robert Muller; Writing by Jason Hovet; Editing by Richard Balmforth)