Yevgeny Urlashov stands in front of his mayoral campaign poster in downtown Yaroslavl in January 2012. (Sergei L. Loiko/Associated Press)

Russia’s highly political law-enforcement authorities took aim Wednesday at their biggest target yet in a campaign to destroy the opposition, launching an attack on the country’s most prominent independent officeholder — a man who had placed his faith in the ballot box.

The mayor of the city of Yaroslavl was taken from his car at 1:20 a.m., and, by first light, investigators were saying they planned to charge him with extortion. By midday, they said they had found stacks of cash in his apartment. By afternoon, claiming that other suspects had agreed to plea bargains, they had labeled Mayor Yevgeny Urlashov the “criminal mastermind” of a plot to shake down a road maintenance company.

“This is about politics,” Urla­shov said at 5:45 a.m. as he was led out of a police van in front of reporters.

Yaroslavl is a venerable, sleepy city of 600,000 on the upper Volga River, a four-hour train ride from Moscow, but Urlashov’s detention and the implausibly rapid development of his case jolted opposition organizers across the country. The mayor was the one nationally known independent politician who had tried to work inside the system and managed to beat President Vladimir Putin’s ruling United Russia party in an election.

Urlashov, 45, won his office in a landslide last year, and he has been the brightest hope of those who sought to use the ballot box to strip influence from Putin’s “vertical of power.”

“The arrest of Urlashov shows once again that it is impossible to reform the system from within,” Ilya Yashin, an organizer of recent public protests, wrote on Twitter. “Real reform requires the removal of Putin from power.”

The criminal charge pending against Urlashov adds to a growing list of cases in which prosecutors answerable to Putin have turned to the courts to silence political opposition.

In Kirov on Wednesday, the judge hearing the case of the anti-corruption blogger Alexei Navalny refused to let the defense present evidence. In Yekaterinburg, Web journalist Aksana Panova pleaded guilty to tax avoidance but not guilty to extortion and theft. In Moscow, the trial of defendants accused of organizing a riot at a 2012 protest is continuing.

In a tweet, Navalny said his chances in court are nonexistent. Others on Twitter noted the disparate developments and pointed out that Wednesday was the novelist Franz Kafka’s 130th birthday.

Extra police flooded the streets of Yaroslavl during the day, but the Interior Ministry said it was only for a drill. Urlashov’s supporters gathered for a protest “get-together” — a “rally” would have required a permit — Wednesday evening.

Urlashov’s detention came only hours after he met with a delegation from the Council of Europe that is visiting Russia to inquire about civil society. One of the visitors is Andreas Gross, who wrote a scathing critique, released last week, of the imprisonment and death of the whistleblower Sergei Magnitsky.

Earlier Tuesday, Urlashov had agreed to join forces with the Civic Platform party organized by Brooklyn Nets owner Mikhail Prokhorov and to appear with Prokhorov in Yaroslavl on Sunday. Prokhorov said Wednesday that having masked police agents take the mayor away in the middle of the night was a move designed to instill fear in Urla­shov and his constituents.

“The seizure of the head of the city who was legitimately elected by the people is a blow at the civil rights and freedom of every citizen of Russia,” he said in a statement. He promised to visit Yaroslavl as planned Sunday.

A month ago, Urlashov said at a rally that he intends to run for governor of the Yaroslavl region next year when gubernatorial elections are reinstated after a 10-year gap. His popularity would make him a formidable candidate in a fair vote. At that rally, he warned that he expected efforts to undermine him, most likely through charges of corruption or drug possession.

“I thought something like this would have happened earlier,” Pyotr Stryakhilev, a local journalist, said in a telephone interview. By waiting until just two months before the September city council elections to move against the mayor, the authorities have made it hard for many to believe that this is not a purely political prosecution, he said.

“There will be every reason to accuse them of all possible sins,” he said. “And Urlashov will be portrayed as a real martyr.”

The mayor insisted Wednesday that the accusations against him are groundless. He has consistently complained about the cost of the road contracts and poor work by the company that holds them. The firm has strong ties to the local United Russia organization and signed the criminal complaint against him. “We do not give up and do not fear,” he told reporters in a phone call from the police station, as reported by the Russian Web site His attorney said that, contrary to police reports, there had been no money in the mayor’s apartment.

“The case against Urlashov looks like a fabrication, was set up as a fabrication and is a fabrication — with 99% probability,” Navalny wrote on his blog. “Urlashov will be on national television, which will be telling lies about this fabricated case and showing all those blockheads from the Investigative Committee trying to destroy his popularity.”

Vladimir Markin, spokesman for the Investigative Committee, Russia’s main federal investigating authority, nearly always declines to discuss ongoing cases. But “testimonies of some of the suspects confirm the suspicions of extortion,” he told the Interfax news agency Wednesday, “and expose the mayor as the criminal mastermind.”