This undated video image provided by the Associated Press Television News shows Leonid Razvozzhayev speaking in an undisclosed location. Ukraine's main opposition party is accusing the government of allowing Russian agents to kidnap the anti-Kremlin activist in the Ukrainian capital and forcibly transport him to Moscow. (AP/AP)

Russian authorities are dramatically escalating the pressure on the opposition that emerged on the streets in December, going so far as to abduct an asylum-seeker in Ukraine in order to force him to testify against others, human rights activists say.

Russian officials deny the abduction. But the United Nations’ refugee agency has demanded an investigation, and the United States has expressed its concern. Leonid Razvozzhayev, the man who was allegedly abducted, is in prison, and Sergei Udaltsov, the leader of their socialist movement, is expected to join him there Friday, charged with inciting mass disorder.

“It’s a turning point,” said Andrei Piontkovsky, a political analyst and critic of Russian President Vladimir Putin. “Either we head toward an openly fascist regime, or we stop it now.”

The pursuit of Udaltsov, the 35-year-old leader of the Left Front bloc, and assertions that Razvozzhayev was abducted in Kiev and then coerced for nearly three days before turning up in custody in Moscow represent a new level of confrontation between Putin and his opponents.

“They are taking tactics developed against terrorists and using them against political opponents,” said Vladimir Pastukhov, a visiting fellow at St. Anthony’s College, Oxford University, who has been an adviser to Russia’s Constitutional Court. “They have started calling everyone who opposes them a terrorist.”

Investigators opened a criminal case against Udaltsov, Razvozzhayev and another activist on Oct. 17 on suspicion of organizing mass protests, based on information from a documentary made by a Kremlin-friendly television station. Udaltsov was questioned and freed on his own recognizance, Razvozzhayev could not be found and the third man, Konstantin Lebedev, was detained.

On Oct. 19, Razvozzhayev was in Kiev seeking asylum from the United Nations High Commissioner for Refugees, according to a statement from the agency. A lawyer from another agency was interviewing Razvozzhayev, who disappeared during a lunch break, leaving his belongings behind.

Russian authorities said Monday that he turned himself in Sunday and voluntarily wrote a 10-page confession, admitting to the charges against him.

On Tuesday night, members of the Public Oversight Commission — citizens who monitor prison conditions — spoke to Razvozzhayev in the Lefortovo prison, once the infamous KGB prison.

They say Razvozzhayev told them he was grabbed on the street in Ukraine, pushed into a van, driven across the border with his head covered and confined to a basement. There he was handcuffed, his legs were taped together, and he was deprived of food and toilet for two days.

His interrogators threatened his wife and children, he said, and he was told he would be injected with a serum that would make him ill and possibly permanently disabled.

“What we heard was shocking,” said Valery Borshchov, head of the commission. “We have a new phenomenon, a new method for fighting dissent.”

On Thursday night, Razvozzhayev finally saw his lawyer, Mark Feygin, who told reporters that he had repeated he was kidnapped.

Udaltsov was a little-known figure on the radical fringe before last winter’s demonstrations against rigged elections and Putin brought him to greater prominence. Since then he has become less radical but still firmly socialist, opposed to capitalism and private ownership of property. Razvozzhayev was a follower who worked as an assistant to Ilya Ponomaryov, a member of the lower house of parliament and the Just Russia party. Ponomaryov has emerged as an opposition leader, too, and some of his supporters suggested that the attack on his aide served as a warning to him as well.

Kirill Rogov, a columnist and political analyst, said that the authorities went after Udaltsov because he was an easy target — more given to intemperate remarks than some — and his example could scare others.

“There is not much sympathy for the left,” he said, “so there is not as strong a reaction from the rest of the world as there would be if they went after a liberal. It’s a start, and then you can widen it.”

Joseph Kruzich, the U.S. Embassy media spokesman, told the Kommersant newspaper that the United States had taken note of “credible reports” from the ­UNHCR and Public Oversight Commission. “We have shared our concerns with the Russian government today and requested that they investigate this matter,” Kommersant quoted him as saying.The Russian Foreign Ministry responded, in a statement to Interfax, “We would recommend that our counterparts pay attention to well-known instances of torture in special CIA jails and to the detention of prisoners at Guantanamo for an indefinite time without trial or investigation, something that tramples upon all standards of international law.”

Putin’s spokesman said the Kremlin had no reason to comment. “It’s a case involving investigative bodies, prosecutors, judges, defense lawyers and human rights activists,” Dmitri Peskov told reporters.

Pastukhov said that Udaltsov frightened the Kremlin more than liberals did. “When you talk about democracy it only touches the elites,” he said, but Putin has been making populist promises about social benefits that will be difficult to deliver, given the nation’s finances. Udaltsov tapped into those aspirations, which touch millions, Pastukhov said.

“I can say fascism is no longer an abstraction,” he said, because government officials widely ignore the law. “It is becoming more and more concrete.”

Two feminist punk rockers went to prison for singing in a cathedral, an offense against good taste but not a crime, he said. Before that, Sergei Magnitsky was charged with theft after he accused officials of the crime. He died in pre-trial detention and then was charged with the crime again, two years after his death.

“The main problem is no one is protected,” he said. “You can destroy anyone’s life as long as he’s not stronger than you.”

The abduction reports, he said, reminded him of the case of Alexander Litvinenko, who died in London in 2006 of polonium poisoning after he fell afoul of Russian officials.

“I don’t see any big difference between the two,” he said. “They think they can act illegally against their opponents overseas, and no one will say no.”

Piontkovsky said now is the time for liberals in the Kremlin to stand up and for the West to make its displeasure clearly known.

“For them, it’s the last chance to swallow these fascist methods or stand up to them,” he said of Kremlin insiders. “They have stayed loyal to Putin in spite of everything, but without a split at the top, it will be very difficult to resist the regime.

“The next days and weeks will be decisive. Either abduction and torture will be more or less accepted by society, or it will be resisted. I’m afraid the liberals in the system will accept it.”