Shortly after Russia shocked the world by attacking Ukraine on Feb. 24, Ilya V. Yashin, a local Moscow councilman and prominent opposition figure, decided it was time to see a dentist.

The Kremlin was in the process of criminalizing criticism of the war, and Mr. Yashin, a very vocal critic, had decided to stay in his home country and continue to oppose President Vladimir V. Putin. Eventually, he reasoned, jail time was highly likely.

“I’m honestly terrified of dentists,” Mr. Yashin said in a recent interview on YouTube, “but I got ahold of myself and did it because I realized that if I ended up in prison, there wouldn’t be any dentists there.”

Two weeks after the interview was published, Mr. Yashin, 39, was indeed arrested. He is now in pretrial detention in Moscow, on charges of “disseminating false information” about the war. He faces a sentence of up to 10 years.

Mr. Yashin’s arrest highlights the rapidly constricting avenues for dissent inside Russia as Mr. Putin cracks down on any divergence from the official narrative of the invasion. Beyond that, it has reignited the debate among the Russian opposition over how leading figures like Mr. Yashin can best serve the cause of undermining Mr. Putin: outside the country they want to reform, or inside a penal colony?

Mr. Yashin remains convinced he made the right choice. “What crime did I commit?” he asked rhetorically in a handwritten letter from prison to The New York Times. “On my YouTube channel, I criticized the special military operation in Ukraine and openly called what is going on a war.”

But some opposition figures disagree, saying that staying and fighting might seem courageous, but that prison is an ineffective platform for pushing reforms.

“Yashin is fearless — he is a fighter, he is brave,” said Dmitri G. Gudkov, a Russian opposition leader who left Russia last year. “I am sure that he will not back down,” he continued. “But I’m just sad that he will waste his life. It’s not understandable.”

Mr. Gudkov went into exile after what he described as “credible threats” that a criminal case against him would result in jail time. He said he had encouraged Mr. Yashin, a longtime friend, to go into exile as well.

Yevgenia M. Albats, a journalist and friend of Mr. Yashin who also decided to stay, took the opposite view, saying it was impossible to engage in politics seriously from abroad.

“You cannot be a Russian politician in New York, in Manhattan,” Ms. Albats said in a phone interview from Moscow. “You cannot call yourself a Russian politician and be in London.” Still, she conceded, “The risks are very high and they are getting higher.”

Mr. Yashin acknowledged as much in the YouTube interview posted shortly before his arrest, with the Russian journalist Yuri Dud. “I understand that each day could be my last one as a free man,” he said.

He later wrote on social media that he believed it was his clear refusal to leave, expressed in that interview, that resulted in his arrest.

In his letter to The Times, which was scanned and sent last week, Mr. Yashin wrote that Russian “prisons are swiftly filling with political prisoners” because Mr. Putin feels threatened.

“These harsh repressions,” Mr. Yashin wrote, “indirectly confirm that the current military campaign is devoid of legitimacy.”

Mr. Yashin knew his outspokenness and his platform would make him a target, and friends agree that his detention was only a matter of time. He had been repeatedly fined for “discrediting” the Russian military — mostly by talking about other wars. In April, he shared a well-known photograph of women protesting the Vietnam War in 1969, saying that the hypocrisy behind the rationale for the war, expressed in the slogan “bombing for peace,” remained present today.

He was also fined in May for citing a condemnation of Russia’s invasion of Afghanistan by Andrei Sakharov, the first Russian to be awarded the Nobel Peace Prize, and the well-known words of a Soviet bard who raised alarm about the invasion of Czechoslovakia in 1968.

After the invasion began in February, he continued to call out Mr. Putin’s government, holding regular livestreams on his YouTube channel criticizing the power of the security services in Russia. He also documented a visit to the penal colony holding the most prominent Russian opposition figure, Aleksei A. Navalny, and made reference to a BBC report about Russian atrocities in Bucha, the basis of his charge for distributing false information.

The only choices open to opposition politicians from Russia today are “emigration or prison,” said Lyubov Sobol, who was forced to emigrate after her boss, Mr. Navalny, survived an attempted poisoning, returned to Russia and was immediately arrested It was on Mr. Navalny’s advice that Mr. Yashin went to the dentist.

Mr. Navalny has remained influential in jail. The large team that he assembled before his arrest has reconstituted abroad. Observers say maintaining such a public profile from prison requires a large apparatus like Mr. Navalny’s; Mr. Yashin has so far been able to smuggle out messages later posted to social media.

Ms. Sobol, a lawyer, said she could not criticize a colleague while he was in jail. But she said no one in Russia could fill in for Mr. Yashin, on YouTube or in the political arena.

“He had a huge YouTube channel, a large audience, which trusted him,” she said of Mr. Yashin, who has 1.3 million subscribers. “I know many people who sent his videos to their grandparents. And they changed their minds about Russian propaganda, because he spoke very simple, bright and good language.’’

“There are no other people” in Russia able to do that right now, she said.

Mr. Yashin became active in politics when he was 17, just as Mr. Putin came to power, and quickly rose to lead Moscow’s chapter of the youth wing of the liberal Yabloko party. When Yabloko reprinted a Russian translation of George Orwell’s dystopian novel “Nineteen Eighty-Four,” Mr. Yashin wrote the introduction, warning that the “era of Big Brother” had begun in Russia.

He eventually became close with the opposition politician Boris Nemtsov, who was shot dead in Moscow in 2015 by assassins believed to be linked to Ramzan Kadyrov, the strongman who has led the Russian region of Chechnya since 2007. Around the time of his murder, Mr. Nemtsov was compiling a report on the involvement of Russian soldiers in the war that had begun in eastern Ukraine in 2014. Mr. Yashin finished and released the report, and became one of the few politicians willing to openly criticize the Chechen leader.

In 2017, Mr. Yashin and fellow opposition candidates won seven out of 10 seats on the local council in the Krasnoselsky district of Moscow.

As council head, Mr. Yashin addressed quotidian concerns: playgrounds, parking, gentrification. He repurposed his official car and driver as a free taxi for the district’s disabled. On YouTube, he delivered regular reports about the council’s achievements and challenges. He called out the corruption of government agencies and subcontractors.

Facing constant scrutiny from the prosecutor’s office, Mr. Yashin stepped down as council head in 2021, said Yelena Kotenochkina, who took over council leadership.

Prosecutors “were constantly checking what we were doing,” she said. Mr. Yashin’s repurposing of his official car prompted an investigation for abuse of power.

In March, another council member, Aleksei A. Gorinov, suggested the district shouldn’t hold a children’s event celebrating the Soviet victory in World War II while children were dying in Ukraine. Ms. Kotenochkina agreed. At the end of April, both were charged under the “false information” law. Ms. Kotenochkina managed to flee to Lithuania; Mr. Gorinov was sentenced to seven years in a penal colony.

Ms. Kotenochkina said the case against her and Mr. Gorinov had been a “hint” to Mr. Yashin that he should leave the country or face prison.

And late one June evening, Mr. Yashin was detained as he walked in a park with a friend, the independent journalist Irina Babloyan. He was accused of disobeying police orders — a bogus charge, insisted Ms. Babloyan — and sentenced to 15 days in jail. As soon as he was released, he was arrested again on the false information charge, and now awaits trial. Last week, Russian authorities labeled him a “foreign agent,” a government label tantamount to enemy of the state.

“Now people see: We are not running anywhere, we stand our ground and share the fate of our country,” he wrote.

“This makes our words worth more and our arguments stronger. But most importantly, it leaves us a chance to regain our homeland. After all, the winner is not the one who is stronger right now, but the one who is ready to go to the end.”

Alina Lobzina contributed reporting.