Sergei Tikhanovsky had just crossed the border into Lithuania after years in a Belarusian prison when he saw himself in a bathroom mirror. He did not recognize the man looking back at him.

“I got scared,” said Mr. Tikhanovsky, a popular YouTuber and opposition leader who lost 132 pounds behind bars. “I had no idea I looked like this.”

For half of his five years in custody, he was held incommunicado in a high-security facility where food was scarce and basic amenities were absent. Now that he is out, freed in June with the help of U.S. diplomacy, he has wasted no time in re-establishing himself as a caustic critic of Belarus’s authoritarian leader, Aleksandr G. Lukashenko.

Mr. Tikhanovsky’s impertinent remarks since his release, including his promise to use medieval torture against Mr. Lukashenko, have caused headaches for his wife, Svetlana Tikhanovskaya. She emerged as Belarus’s top opposition figure in his absence and runs a government in exile from Lithuania.

“I’m teaching him to be more careful about the words you say,” Ms. Tikhanovskaya, 42, said as her husband has tumbled back into the political fray after being almost entirely cut off from the world for more than two years.

But she and Mr. Tikhanovsky, 46, are united as sworn enemies of Mr. Lukashenko, an international pariah who unleashed a reign of terror in 2020 in response to protests against his fraud-tainted election victory.

When Mr. Tikhanovsky was jailed that spring just before he was to file papers to run for president, Ms. Tikhanovskaya, then a stay-at-home mother, stepped in and got on the ballot. She galvanized Belarusians fed up with the oppressive rule of Mr. Lukashenko, the closest ally of President Vladimir V. Putin of Russia and a key enabler of his war against Ukraine.

After Ms. Tikhanovskaya fled to Lithuania, she successfully lobbied for international sanctions against the Belarusian government over its suppression of the protests. She became a well-connected politician who had European leaders on speed dial, lobbying energetically for the release of almost 1,200 Belarusian political prisoners.

All the while, her husband was suffering under increasingly dire conditions in prison.

At first after his detention, Mr. Tikhanovsky was permitted to write letters to family and supporters. But he disappeared from view at the end of 2021, when he was found guilty of organizing mass unrest and sowing hatred and sentenced to 18 years. It was the longest term given to any opponent of Mr. Lukashenko, who has led Belarus for 31 years.

Cold and hunger were a constant in prison. Mr. Tikhanovsky was kept in solitary confinement, and he could not see a lawyer or get any news from home. When he asked his jailers why he was not getting any letters, even from his two children, he was told: “No one is writing to you. Everyone forgot about you.”

In February 2024, Mr. Tikhanovsky was listening to a prison radio when the death of Aleksei A. Navalny, a Russian opposition leader, was announced. Mr. Navalny, 47, had died in prison in murky circumstances.

“I thought I would end up like him,” Mr. Tikhanovsky said in an interview last month in Vilnius, still looking sickly thin four weeks after his release.

The winds began to change, however, at the end of last year. Mr. Lukashenko started to release political prisoners, mostly ordinary Belarusians, in apparent anticipation of a thaw with the incoming Trump administration.

Mr. Tikhanovsky said he sensed a deal was possible in President Trump’s first month in office, when officials from the KGB, the Belarusian security apparatus, tried to persuade him to petition for a pardon, he said.

In late June, Keith Kellogg, Mr. Trump’s special envoy for Russia and Ukraine, visited Minsk, Belarus’s capital. Mr. Lukashenko pardoned 14 political prisoners, including Mr. Tikhanovsky, the highest-profile figure to walk free during the president’s tenure. Mr. Lukashenko’s office released photographs of the president shaking hands with Mr. Kellogg.

“Donald Trump practically saved my life,” Mr. Tikhanovsky said, “and I will always be grateful for it.”

His wife said she was painfully aware that Belarus, a nation of nine million people, “probably doesn’t exist for President Trump as such,” registering only for its support of the Kremlin. But she said the prisoner releases offered a tangible win for the Trump administration as it has struggled to achieve gains in Ukraine and Gaza.

On Sunday, Mr. Trump posted a message on his social media site thanking Mr. Lukashenko for the prisoner releases and saying he hoped there would be more.

Mr. Tikhanovsky said he believed that U.S. diplomats “could secure the release of 1,000 people” in Belarus as it seeks relief from sanctions. His wife is more cautious. The Lukashenko government keeps jailing its critics and “seeking the release of people without putting an end to the repressions is a vicious circle,” she said.

The Tikhanovskys are still unsure why Mr. Lukashenko pardoned Sergei and not prominent faces from the protests like Maria Kolesnikova or Viktor Babariko. But Mr. Tikhanovsky supports a long-rumored theory that the Belarusian leader believed that freeing him would generate a split in the opposition ranks because of Mr. Tikhanovsky’s flamboyant personality and hot temper.

Mr. Tikhanovsky said he had no intention to divide his allies. But he has rejected what U.S. officials said was a request by Mr. Lukashenko to keep a low profile, at least initially, quickly giving interviews and speeches that caused controversy.

At one rally, Mr. Tikhanovsky, who was not around to witness the large-scale repression of ordinary Belarusians over any display of disloyalty to Mr. Lukashenko, derided some activists for hiding behind face masks.

The latest storm stirred by Mr. Tikhanovsky involved a poorly worded idea to create “islands” of Belarusian communities in exile, which caused a rift with Lithuania. In that country, any remarks on the issue of the nation’s sovereignty or possible autonomy for minorities are particularly fraught, bringing painful memories of the Soviet occupation that lasted until 1991.

Mr. Tikhanovsky conceded that some of his remarks had been problematic but attributed them to the challenge of getting caught up on five years of political discussions in a matter of weeks. His wife acknowledged that his statements had hurt ordinary Belarusians and stirred trouble abroad.

When he is not setting political fires, Mr. Tikhanovsky is embracing domestic life in a way he never did before his incarceration, his wife said, spending more time with their children and even cleaning up around the house.

He has promised not to meddle in the work of his wife’s well-oiled government in exile. He does not even have a pass for her office. He plans to run a separate operation targeting Belarusians at home, but he does not yet know what form it will take.

“I fully intend to break down the regime, and I have my own way,” he said. “I’m going to energize the Belarusians for a victory.”