Independent Russian vote monitor Golos has been labelled a foreign agent a month ahead of parliamentary elections.
Golos detailed cases of fraud across Russia in the 2011 parliamentary vote and its head has vowed it will continue its work in the September vote.
President Vladimir Putin's United Russia has declined in popularity but is aiming to maintain its majority.
Opposition figures have been targeted, with Kremlin critic Alexei Navalny in jail and allies barred from running.
Formed in 2000 as a non-government organisation, Golos (which means "voice" and "vote") was declared a foreign agent in 2013 but then became an unregistered civil movement.
It has now become the first group on a new list of foreign agents that covers unregistered groups, apparently because of funding from an Armenian citizen.
"This is an attempt to prevent Russian citizens from exercising their right," said Golos co-chair Grigory Melkonyants, who pledged to continue as normal.
The Organisation for Security and Cooperation in Europe (OSCE) said recently it would not send observers for next month's elections due to restrictions imposed by the Russian authorities.
