All telephones in the office were disconnected.
Memorial head Sergei Khakhayev told RFE/RL's Russian Service that he believes all of the hard discs were removed from the group's computers; he said no one was allowed inside during the operation.
Documents including a history of political repression in the Soviet Union and a computer-based tour of Stalin-era forced work camps or gulags were taken away, staff member Yuly Rybakov told Reuters.
"All these years of work are now in the hands of the investigators and we do not know when and in what condition it will be returned," Rybakov said. "This is an act of repression against our organization."
A spokesman for the Russian Prosecutor-General's main investigative unit was quoted by Interfax news agency as saying the search was part of a probe about a local newspaper article. No other details were given.
Human Rights Watch (HRW) said the Russian government should immediately investigate the raid. In a statement, HRW also called on the government to ensure the speedy and safe return of all seized equipment and documents.
“This outrageous police raid on Memorial shows the poisonous climate for nongovernmental organizations in Russia,” said Allison Gill, Moscow office director at HRW. “This is an overt attempt by the Russian government to suppress independent civic activity and silence critical voices.