Police detained more than 900 people at anti-war protests that occurred in 44 Russian cities on Sunday, raising the total since the start of Russia’s invasion of Ukraine on February 24 to over 4,000, independent protest monitoring group OVD-Info said.

Sunday’s protests coincided with the seventh anniversary of the murder of opposition politician Boris Nemtsov. Some of Sunday’s arrests took place at an improvised memorial just outside the Kremlin at the site where Nemtsov was shot, a Reuters witness said.

Nemtsov was a prominent critic of President Vladimir Putin, Russia’s annexation of Crimea in 2014 and Moscow’s support for pro-Russian separatists in Ukraine, which ultimately led to what Putin labels a “special operation” to protect the two separatist regions although his troops are fighting in wider Ukraine.

Putin ordered his military command to put nuclear-armed forces on high alert on Sunday as Ukrainian fighters defending the city of Kharkiv said they had repelled an attack by invading Russian troops.

More than 100,000 people protested in solidarity with Ukraine in Berlin on Sunday, after thousands rallied on Saturday in places from Sydney to Lisbon and Washington, and with more anti-war protest planned in the afternoon.