By David M. Herszenhorn
October 17, 2012

MOSCOW — Russian authorities on Wednesday arrested one of the country’s most prominent leaders, accused him of plotting to organize mass riots and said he could face terrorism chargespolitical opposition.

The arrest of Sergei Udaltsov, the buzz-cut, black-clad leader of the Left Front, a radical socialist group, seemed to sharply accelerate the government’s efforts to bring serious criminal cases — with the prospect of long prison terms — against critics of President Vladimir V. Putin.

Mr. Udaltsov is a fixture at antigovernment rallies, is arrested frequently and has had numerous short stays in prison, often for administrative offenses like disobeying the police.

But the new accusations are far more serious, stemming from a recent documentary on the pro-government NTV channel that appeared to show him discussing efforts to topple the Russian government with an official from neighboring Georgia, as well as appealing for financial support.

A spokesman for Russia’s top federal investigator, Aleksandr I. Bastrykin, even raised the prospect of life imprisonment.

“I would like to draw the attention of those who thought that in our country it is possible with absolute impunity to organize mass disorders, to plan and prepare terrorist acts and other actions threatening the life and health of Russians,” the spokesman, Vladimir Markin, said in a televised statement. “You underestimate the professionalism of Russia’s special services.”

He added that Russian law “envisages for such offenses punishment up to life imprisonment.”

Although many of the best-known political opposition leaders in Russia have come under the pressure of criminal investigations in recent months — notably, the corruption fighter Aleksei Navalny and the television personality Kseniya Sobchak — there has yet to be a major prosecution. That seems about to change.

Mr. Udaltsov poses a particular threat to the Kremlin because his views are popular among members of the Communist Party, which retains a strong nationwide infrastructure and often finishes second to the governing party, United Russia, in major elections.

In a nod to Mr. Udaltsov’s popularity on the left, the longtime leader of the Communist Party, Gennady A. Zyuganov, demanded Wednesday that the authorities show that their case is not just a matter of political retribution based on a sensational documentary.

“I would like for the prosecutor and the investigative bodies not to present this fake of NTV, which smells rotten,” Mr. Zyuganov said, “but to present real facts.”

After the documentary was released this month, Mr. Udaltsov immediately denounced it in a Twitter post as “dirt and lies,” calling it a “provocation whose ultimate objective is to justify my arrest.”

On Wednesday, his arrest was carried out in theatrical fashion. Mr. Udaltsov was escorted from his apartment by a squad of commandoes in black masks, though they did not bother to handcuff him.

Video showed Mr. Udaltsov flashing a victory sign as he emerged from the building, walking past two elderly women, seated on a bench with their hands folded in their laps, whose faces did not register the least bit of surprise.

Mr. Udaltsov has previously cooperated with the authorities by appearing for questioning when summoned and, on one occasion, notifying them in advance that he would not appear as requested because the appointed time conflicted with a large protest of the government that he was helping to lead. He said he needed to fulfill duties set forth in the event permit.

But if the dramatic nature of his arrest seemed unnecessary, it was not unusual for Russia and appeared intended to underscore the seriousness of the authorities in pursuing the new criminal case.

Nevertheless, the police released Mr. Udaltsov on Wednesday after barring him from traveling outside the country, a common restriction on subjects under investigation. They said a decision would be made shortly on what formal charges, if any, will be brought.

An aide to Mr. Udaltsov, Konstantin Lebedev, who was shown with him in the NTV documentary, was also arrested Wednesday.

The authorities said they also planned to question a third man shown in the documentary, Leonid Razvozzhaev, who is an aide to Ilya V. Ponomarev, a member of the lower house of Parliament. Mr. Ponomarev is also a leader of the Russian political opposition.

In the documentary, called “Anatomy of a Protest — 2,” Mr. Udaltsov and the other men are shown, apparently recorded by a hidden camera, meeting with a man identified as Givi Targamadze, the chairman of the defense committee in the Georgian Parliament.

Mr. Markin, the spokesman for the federal Investigative Committee, said that the authorities had used “phonoscopic research” to confirm the identities of Mr. Udaltsov and the other men. He said the meeting took place in June in a residential neighborhood of Minsk, the capital of Belarus.

Mr. Udaltsov’s wife, Anastasiya, posting on Twitter, urged his supporters to speak out against his arrest. She also noted that Mr. Bastrykin, the chief federal investigator and a close ally of Mr. Putin, had been accused earlier this year of threatening to kill a newspaper editor in response to an article that he did not like. Mr. Bastrykin apologized, but there was no formal investigation of the episode.

Andrew Roth contributed reporting.