Leonid Razvozzhayev spoke quickly from the bed of his one-man jail cell. "I'm scared to stay here alone," he told the group of prison rights activists who had come to visit him. "I'm afraid those men – the ones who took me me, who tortured me – will come back."

Razvozzhayev, a 39-year-old anti-Putin activist and aide to an opposition MP, flew to Ukraine last week amid a spiralling crackdown on Russia's opposition. Just a day after he arrived, Russian prosecutors opened a criminal investigation into him, the first step to bringing charges for allegedly organising mass unrest and a potential 10-year jail sentence. Thus Razvozzhayev began the long process of bidding for political asylum – and in the midst of it disappeared, only to turn up at a Moscow prison two days later.

Razvozzhayev's story – one of alleged abduction, torture, death threats and forced confessions – has sent a bolt of fear and anger through those opposed to Russia's powerful president. It is the latest, and perhaps most convoluted, tale to emerge from the Kremlin's attempt to put down the opposition that emerged to challenge Putin's return to power earlier this year.

"He's not sure what day it is – he's in such a state, under such pressure," said Zoya Svetova, one of the five activists who visited Razvozzhayev on Tuesday. Svetova is a journalist for the New Times, an opposition magazine, in which she described her conversation with Razvozzhayev.

Razvozzhayev told the group how he had left Russia for neighbouring Ukraine on 15 October. The next day, prosecutors in Moscow opened a criminal investigation into him and two fellow activists in the socialist Left Front – its leader, Sergei Udaltsov, and Konstantin Lebedev. They were suspected of plotting mass unrest and terrorist attacks in Russia, investigators said, citing a propaganda report on state-run television that claimed to show the men plotting revolution.

Fearing the worst if he were to return to Russia, Razvozzhayev, an aide to outspoken parliamentary deputy Ilya Ponomaryov, decided to seek political asylum and approached the United Nations high commissioner for refugees (UNHCR) on 19 October. They directed him to a partner organisation for legal counselling. The talks were long. He went out for a sandwich – and never came back.

Razvozzhayev said four men were waiting for him at the exit. "They threw me in a minivan," the New Times quoted him as saying. "They put a tight black hat on me, so I couldn't see anything, and tied my arms and legs up with tape. And we were off." UNHCR issued a statement saying it was "deeply concerned" about his disappearance.

They drove for hours in near total silence, Razvozzhayev said. At what he believes was the Russia-Ukraine border, he was put into another minivan and driven further. Once they stopped, he was handcuffed and dragged into a cellar and interrogated for two days, he said, alleging psychological torture.

"For two days they didn't let me go to the toilet. I didn't eat, I didn't drink the whole time," Razvozzhayev told the prison rights activists. The handcuffs never came off, he added. The unidentified men pressured him into signing a confession and implicating his fellow activists, Razvozzhayev alleged.

"They told me: if you don't answer our questions, then your children will be killed," he told the group. "I agreed to say what they wanted." He was then taken to the headquarters of the investigative committee in Moscow, a body akin to the FBI.

Video obtained by Life News, a tabloid, and aired on its website on Sunday evening showed Razvozzhayev being transferred from the building to a waiting police van, shouting "Tell them I've been tortured". On Monday, the committee issued a statement saying Razvozzhayev had wilfully turned himself in. Officials have denied all claims of torture.

The investigative committee charged Razvozzhayev on Tuesday and remanded him in custody for two months. Udaltsov is due to be charged on Friday, it said. The men face up to 10 years in prison if found guilty.

"The massive persecution of dissidents in Russia is continuing," the opposition said in a joint statement on Wednesday. ""The Russian authorities have turned to methods of direct pressure of force against its opponents." Activists held a day-long protest outside the headquarters of the federal security service, the main successor to the KGB, on Wednesday.

"One week ago, we didn't think that a person could be abducted and forced into a confession," Svetova told the Guardian. "We didn't think it was possible to open a criminal case because of a TV programme. This didn't even exist in the Soviet Union. Who knows what's next."

On Tuesday, the Russian parliament passed the latest in a string of laws that activists warn are designed to crack down on the opposition. The law, which must be signed by Putin before coming into force, expands the definition of high treason to include vague wording that could be used against any Russian who speaks to a foreigner, activists warned.