More than 2,500 protesters were detained during anti-war demonstrations across 49 Russian cities, according to an independent Russian-based protest monitoring group.
Thousands of protesters against the Russian invasion of Ukraine chanted "No to war!" and "Shame on you!" according to videos posted on social media by opposition activists and bloggers. The OVD-Info protest monitoring group said 2,502 people had been arrested.
Dozens of protesters in the Urals city of Yekaterinburg were shown being detained. One protester there was shown being beaten on the ground by police in riot gear. A mural in the city showing President Vladimir Putin was defaced.
Reuters was unable to independently verify the footage and photographs on social media or to reach Russia's interior ministry for comment.
"The screws are being fully tightened – essentially we are witnessing military censorship," Maria Kuznetsova, OVD-Info's spokesperson, told Reuters by telephone from Tbilisi.
"We are seeing rather big protests today, even in Siberian cities where we only rarely saw such numbers of arrests."
The ministry warned on Saturday that any attempt to hold unauthorized protests would be prevented and the organizers held to account. It did not immediately publish data on arrests.
Police used loudspeakers to tell a small group of protesters in Khabarovsk: "Respected citizens, you are taking part in an unsanctioned public event. We demand you disperse."
Some Russian state-controlled media carried short reports about the protests.
Russia's RIA news agency said the Manezhnaya Square in Moscow, adjoining the Kremlin, had been "liberated" by police who had arrested some participants of an unsanctioned protest against the military operation in Ukraine.
Church support
RIA also showed footage of what appeared to be supporters of the Kremlin driving along the embankment in Moscow with Russian flags and displaying the "Z" and "V" markings used by Russian forces on tanks operating in Ukraine.
Patriarch Kirill, head of the Russian Orthodox Church, said Russian values were being tested by the West, which offered only excessive consumption and the illusion of freedom.
Putin, Russia's paramount leader since 1999, calls the invasion, launched on Feb. 24, a "special military operation." He says it is aimed at defending Ukraine's Russian-speaking communities against persecution and preventing the United States from using Ukraine to threaten Russia.
The West has called his arguments a baseless pretext for war and imposed sanctions that aim to cripple the Russian economy. The U.S., Britain and some other NATO members have supplied arms to Ukraine.
Jailed Kremlin critic Alexei Navalny had called for protests on Sunday across Russia and the rest of the world against the invasion.
About 2,000 people attended an anti-war protest in Kazakhstan's biggest city Almaty, according to videos posted on social media. Reuters was unable to independently verify the posts.
The crowd shouted slogans such as "No to war!" and obscenities directed at Putin while waving Ukrainian flags.
Blue and yellow balloons were placed in the hand of a statue of Lenin towering over the small square where the rally took place.
The Russian state polling agency VTsIOM said Putin's approval rating had risen 6 percentage points to 70% in the week to Feb. 27. FOM, which provides research for the Kremlin, said his rating had risen 7 percentage points to 71% in the same period.