The civil rights portal OWD-Info provides independent information on the number of people arrested during protests in Russia - its website has now been blocked.
It was no longer available on Saturday in the capital, Moscow.
"We do not know the reason for the blocking," announced the portal in the Telegram news channel.
One was not informed by the authorities.
According to the Interfax news agency, the blocking was made by court order.
The Russian media regulator Roskomnadzor confirmed the move, referring to the decision of the judges, who found that information from Owd-Infor "is aimed at justifying the activities of extremist and terrorist groups".
The activists now want to take action against the lockdown.
In several social networks, however, they could continue to provide information about arrests, for example.
However, there was access to the OWD-Info page via a protected network connection (VPN).
Blocked websites can be accessed with such VPN connections.
Thousands of Internet sites have been blocked in Russia, many of them from opposition members, but also from independent and critical media.
The authorities justify the bans with violations of Russian law.
Critics, on the other hand, complain about censorship.
Freedom of expression in the largest country in the world in terms of area is being increasingly restricted.
In addition, many non-governmental organizations and the media are classified as “foreign agents” in Russia, which they criticize as stigmatization.
Two groups have now been added to the controversial register, campaigning for lesbians, gays, bisexuals, transgender and intersex people (LGBTI).