Locked in an Arctic prison, Aleksei A. Navalny is likely to have spent his final days in some of the most inhumane conditions within Russia’s extensive penitentiary system, according to five men who have served sentences in the same penal colony as the Russian opposition leader.

The men described in phone interviews unbearable cold, repulsive food, unsanitary conditions and beatings in Penal Colony No. 3 of the remote Yamalo-Nenets region, where Mr. Navalny arrived in December to serve out the remainder of his 19-year old prison sentence. The former inmates said the conditions were especially brutal in the solitary cells where Mr. Navalny is believed to have been confined on the day he was pronounced dead.

But what made the prison, known as IK-3 or the Troika, dreaded even by Russia’s hardened inmates was the exceptional psychological pressure and loneliness, they said. It was a system devised to break the human spirit, by making survival depend on total and unconditional obedience to the will of guards.

“It was complete and utter annihilation,” said a former inmate named Konstantin, who spent time in the prison’s solitary confinement cells. “When I think about it, I still break into cold sweat,” he said, adding that he has struggled with mental illness since his release.

The New York Times interviewed four men who had finished serving sentences in the Troika in the past decade, some as recently as weeks before Mr. Navalny’s arrival. The Times has also spoken to one person who was in the colony at the time of Mr. Navalny’s death, as well as a friend of a former recent inmate. Their full names and some personal details are being withheld to protect them from retribution.

Mr. Navalny, who will be buried on Friday in a Moscow cemetery, described his time in the Troika in occasional social media posts with the sarcasm, wit and understatement that had helped turn the former blogger into the face of opposition to President Vladimir V. Putin.