Lithuanian Laima Andrikiene, the deputy chairwoman of the European Parliament's Human Rights subcommittee, was at Moscow's Triumph Square at the Strategy 31 demonstration attended by some 400 protesters.
Andrikiene was visiting Russia along with three other members of the European Parliament's Human Rights subcommittee. She attended the rally at the invitation of opposition United Citizens' Front member Andrei Nekrasov.
She said was surprised by the large number of police at the protest and the police officers' behavior in arresting some 70 demonstrators, who she said were picketing "peacefully."
Andrikiene said she plans to include the incident in a human rights report that will be issued by the European Parliament later this year.
The demonstration was one in a series held on the 31st day of the months with that many days. Protesters want to draw attention to Article 31 of the Russian Constitution, which guarantees freedom of assembly.
Despite the constitutional guarantee, authorities have repeatedly refused to sanction Strategy 31 demonstrations.
Among those arrested on August 31 were opposition leaders Boris Nemtsov, Ilya Yashin, Eduard Limonov, and Sergei Udaltsov.