Lev Ponomaryov told the ITAR-Tass news agency the raiders roughed him up and locked him out of the premises, which housed the offices of the For Human Rights organization.
The property management firm told ITAR-Tass the offices belonged to the city and the For Human Rights lease had expired in February.
Ponomaryov said the lease was the subject of an ongoing legal battle.
"We really have a property dispute, but until there is a court ruling, we have the right to remain in the premises," he said. "Officials with police broke in. They changed the locks and demanded we leave the office."
For Human Rights is an umbrella organization for 120 separate groups from across Russia.