Body parts belonging to an estimated 34 people have been unearthed along with a rusty pistol at a 19th-Century Moscow mansion undergoing renovation.
Josef Stalin presided over a period of purges and mass killings
Workers found the remains at a city centre site once owned by a tsarist-era noble family, the Sheremetyevs.
Forensic experts are trying to identify the bodies, some of which appear to date to the 1930s, an official said.
Thousands of perceived opponents and rivals of the Soviet leadership were killed by secret police in that period.
The killings reached their climax in 1937 and 1938, when historians say some 1.7 million people were arrested by the security services, at least 818,000 of whom were then shot.
The era, under the leadership of the dictator Josef Stalin, came to be known as the Great Terror.
Some of the remains, discovered in the basement of one of the buildings on the Moscow estate, had gunshot wounds to the back of the head, police spokesman Yevgeny Gildeyev told the Associated Press news agency.