“They beat me with nightsticks, for a long time,” Mr. Lapunov said. “They beat me with their hands and feet. When I left Chechnya, I was literally crawling.”

While detained at the police station, Mr. Lapunov said, he heard screams of other men being tortured. He said he was not subjected to electrical shocks, but did see at the station a hand-cranked electrical generator, consistent with the device other gay men said had been used to torment them.

His family had reported him missing after he disappeared. His lawyer, Vladimir Smirnov, said he had documented his bruises with photographs after his release, could identify the police station where he was held and knew the names or nicknames of about a dozen police officers involved.

Mr. Smirnov said that filing the report with prosecutors in the North Caucasus, the region in southern Russia that includes Chechnya, put his client at grave risk, not least because the police have so far refused to provide him protection as a witness.

“To appeal to the authorities, and to appear here before you, required tremendous courage,” said Tanya Lokshina, Russia program director for Human Rights Watch.

Initially, the legal team did not publicize the effort, hoping prosecutors would push ahead with an investigation. But the case stalled. Mr. Smirnov said prosecutors declined to escort Mr. Lapunov to Grozny, the Chechen capital, to allow him to identify the police station where he was detained, and the perpetrators.

Mr. Smirnov said that his client was now at risk — he has “a target on his forehead,” Mr. Smirnov said — without results to show for it.