Tor, which is partly funded by the U.S. and Swedish governments, among other supporters, is one of the world’s preeminent anti-censorship tools, and it is used by activists and journalists around the world. As it is the gateway to the “dark web” of sites that can’t be accessed through standard browsers, the network is also used by criminals and the law enforcement agencies that investigate their activities. Tor’s name is short for “The Onion Router”—a reference to the fact that the user’s web traffic is disguised in layers of encryption as it is routed through multiple nodes to disguise its origin.

The service has been hugely popular in Russia, where the government maintains a sprawling list of websites that have been banned for everything from advocating protests to promoting what the Kremlin calls “gay propaganda.” According to the Tor project, 15% of its users came from the country, making it second only to the U.S., which accounts for 21% of Tor’s user base.

According to the Seattle-based project, some Russian internet service providers started blocking Tor’s website a week ago. On Wednesday the ban became official, with the site being officially targeted by Roskomnadzor, the Russian media regulator. The state censor said it had ordered the Tor site’s blockage because it contains “information that ensures the operation of tools that provide access to illegal content.”

Indeed, the Putin administration’s long-standing war against online free expression has since 2017 included a ban on services that allow anonymous web surfing. Until now, this has mainly spelled trouble for the virtual private network (VPN) services that allow people to access banned websites and to evade censorship.

Blocking Tor’s website does not mean that those who have already downloaded Tor can no longer use it. The project also responded to Moscow’s move by directing would-be Russian users to a mirrored version of the site, run by the U.S.-based Electronic Frontier Foundation, the world’s top digital rights group.

In a Wednesday blog post that went up shortly before the Russia-wide ban came into effect, the Tor project urged its army of volunteers to set up more so-called Tor bridges—nodes on the network that aren’t listed in Tor’s public directory of nodes, making them particularly useful for people living under oppressive regimes who don’t want to be seen connecting to a known Tor node.

“We will need many more bridges to keep Russians online,” wrote Gustavo Gus, Tor’s community team lead.

The Tor site blockage comes near the end of a year in which Russia’s online crackdown has gone into overdrive. In March, Russia deliberately slowed down Twitter for local users because the company refused to remove tweets that contained banned material. And in September, with elections looming, the Russian authorities successfully pressured Google and Apple into removing opposition apps that showed people how to tactically vote against the ruling United Russia party.

This was possible in part because these Big Tech companies had given in to official demands that they set up Russian offices to keep operating there—so when pressure needed to be applied, the authorities were able to send armed men to Google’s offices and to threaten local employees with prosecution. Apple even went so far as to disable its Private Relay feature in Russia, removing another tool that people could use to conceal their activities from government surveillance.

The Russian government is still trying to get Meta/Facebook, TikTok , Twitter, and others to set up local offices. If they don’t give in by next year, they might face fresh restrictions in Russia.