Mr. Urlashov, speaking on television, denounced the corruption allegations as retribution. “Nothing has been confiscated,” he said. “They looked, made a search in practically all of my deputies’ offices, found nothing.” He said that he had been set up by a man who is a member of United Russia, the party that nominated Mr. Putin for president.

“The man from whom I did not accept city cleaning, road repairs, wrote a report that I extorted bribes from him,” he said. “I was warned that I would be removed one way or another.” He added: “This is a political game, which is linked to the September 8th election. This is very unpleasant to me because I am a patriot of my country.”

But even as Mr. Urlashov issued his forceful denials, federal officials were declaring his guilt. “Testimonies of some suspects confirm the suspicions of extortion and expose the mayor as the criminal mastermind,” Vladimir Markin, the spokesman for the federal Investigative Committee, said. “Besides, one of the suspects has expressed his wish for a plea bargain, which implies the full acknowledgment of one’s guilt and active cooperation with the police.”

Officials said that they had also seized $500,000 from Mr. Urlashov’s apartment, and an additional $200,000 from the office of Mr. Urlashov’s press secretary, Svetlana Yefimova.

Mr. Urlashov was arrested along with a deputy mayor, Dmitry Donskov; the head of the city agency for municipal contracts, Maksim Poykalaynen; and a mayoral adviser, Aleksei Lopatin. They were charged with large-scale extortion and bribery — charges that carry a potential jail sentence of 8 to 15 years.

Mr. Prokhorov, who ran for president last year, said that he would still make a planned trip to Yaroslavl on Saturday in support of Mr. Urlashov’s and Civic Platform’s other candidates in the coming regional parliamentary elections.

In his statement, Mr. Prokhorov said the highly publicized detention of Mr. Urlashov “was staged with one purpose: to intimidate Yevgeny himself as well as all independent political figures and simply active citizens.”