MOSCOW — The lead lawyer defending the Russian opposition leader Aleksei A. Navalny in an extremism case that could outlaw Mr. Navalny’s opposition movement was arrested on Friday, the latest instance of a remarkable escalation by the Kremlin in its long-running campaign to stifle dissent.
The lawyer, Ivan Pavlov, was detained after Russia’s Federal Security Service, or F.S.B., raided his Moscow hotel room at 6:40 a.m., his colleagues said. He stood accused of disclosing details of a law-enforcement investigation unconnected to Mr. Navalny and faced three months of prison time. Mr. Pavlov’s colleagues said agents also searched their group’s St. Petersburg offices and broke down the apartment door of their technology manager.
Mr. Pavlov, one of Russia’s best-known human rights lawyers, has frequently represented high-profile defendants in cases involving the F.S.B., a successor to the K.G.B. that wields enormous influence in Russia. His arrest shook Russia’s activist community because lawyers have, for the most part, been able to continue to operate even as the authorities have intensified their crackdown on the opposition.
“Ivan’s arrest is connected to his professional activity,” a group of lawyers said in an open letter Friday. “We believe that these actions by law enforcement are aimed exclusively at scaring Ivan and his colleagues in order to force them to reject an active position in defending their clients.”
A judge later released Mr. Pavlov but barred him from using the phone or the internet.
Dmitri S. Peskov, the spokesman for President Vladimir V. Putin, said he did not know the reasons for Mr. Pavlov’s arrest and could not comment on it. But he denied that the recent spate of law-enforcement pressure on opposition figures, activists, journalists and now lawyers was all part of a single campaign against Kremlin critics.
“This is not all part of a unified trend,” Mr. Peskov told reporters. “These are different episodes and different cases.”
Mr. Pavlov runs a legal rights group called Team 29, named after the article of the Russian Constitution that guarantees the freedom of thought and speech. The group’s clients include Ivan Safronov, a former journalist accused of spying for NATO last year.
Team 29 said that Mr. Pavlov was being investigated for allegedly disclosing classified details of the Safronov case to the news media.
But the raids came just four days after Mr. Pavlov took up the defense of Mr. Navalny, Russia’s most prominent opposition figure. On Monday, Team 29 announced it would be representing Mr. Navalny’s organizations in a lawsuit filed in April by Moscow prosecutors to have the organizations outlawed as extremist groups.
“Under the guise of liberal slogans,” prosecutors said, “these organizations are busy forming conditions for destabilizing the social and sociopolitical situation.”
Mr. Pavlov said on Monday that his team would disclose as much information as possible about the extremism case against Mr. Navalny’s Anti-Corruption Foundation and his regional offices, even though the authorities had classified the evidence as a state secret.
“We know very well what a state secret is, and we know how to work with this,” Mr. Pavlov said. “We also know which information cannot be classified under any circumstances.”
Mr. Navalny returned to Russia in January after recovering from a poisoning that Western officials believe was an assassination attempt by the Russian state. Since then, the Russian authorities have entered a new phase of their yearslong campaign to repress the opposition, increasing their pressure on journalists and jailing or forcing into exile Mr. Navalny’s associates. Mr. Navalny was sentenced to a two-and-a-half year prison term in February for violating parole for what rights groups say was a politically motivated conviction for embezzlement.
Anticipating that Mr. Navalny’s movement would soon be outlawed as extremist, the opposition leader’s associates on Thursday said they were shutting down their nationwide network of 40 regional offices.
The crackdown on dissent has accompanied rising tensions between Russia and the West, leaving regular Russians increasingly isolated from the outside world. In response to the April 15 American sanctions against Russia over hacking and other “harmful foreign activities,” Russia countered by, among other things, prohibiting the U.S. Embassy in Moscow from employing people who are not American citizens.
The U.S. Embassy said on Friday that the move had forced it to lay off three quarters of its consular staff. As a result, the embassy said, it would stop processing nearly all nonimmigrant visa requests in Russia, such as for tourism or work travel.
Ivan Nechepurenko contributed reporting.