OVD-Info says arrests were made in 49 cities including the Pacific port city of Vladivostok and the Siberian city of Irkutsk.
More than 3,500 people were arrested at protests across Russia on Sunday against President Vladimir Putin’s invasion of Ukraine, according to data provided by the interior ministry.
Thousands of protesters chanted “No to war!” and “Shame on you!”, according to videos posted on social media by opposition activists and bloggers.
Dozens of protesters in the Urals city of Yekaterinburg were shown being arrested. 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.
The footage and photographs on social media could not be independently verified.
Russia’s interior ministry said 1,700 people had been arrested in Moscow, 750 in St Petersburg and 1,061 in other cities. The ministry said 5,200 people had taken part in the protests.
The OVD-Info protest monitoring group said it had documented the arrest of at least 2,578 people in 49 different cities.
The arrests on Sunday brought the total number of people held in anti-war protests since the invasion began on February 24 to more than 10,000, OVD-Info said.
“The screws are being fully tightened – essentially we are witnessing military censorship,” Maria Kuznetsova, OVD-Info’s spokeswoman, told Reuters news agency by telephone from Tbilisi.
“We are seeing rather big protests today, even in Siberian cities where we only rarely saw such numbers of arrests.”
A video posted on social media showed a protester in a square in the far eastern city of Khabarovsk shouting: “No to war! How are you not ashamed?” before two policemen arrested him.
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.”
Al Jazeera was not able to independently verify the post.
Jailed Kremlin critic Alexey Navalny had called for protests on Sunday across Russia and the rest of the world against the invasion.
“Because of Putin, Russia now means war for many people,” Navalny said on Friday. “That is not right: It was Putin and not Russia that attacked Ukraine.”
On February 24, Putin ordered what he calls a “special military operation” to defend Russian-speaking communities against persecution in Ukraine and to prevent 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 US, United Kingdom and other NATO members have supplied arms to Ukraine.
Putin’s approval ratings have jumped in Russia since the invasion, according to Moscow-based pollsters.
The president’s rating rose six percentage points to 70 percent in the week ending February 27, according to state pollster VsTIOM. The FOM pollster, which provides research for the Kremlin, said Putin’s rating had risen seven percentage points to 71 percent in the same week.
A number of international broadcasters, including the BBC, CNN, Italy’s RAI and Germany’s ARD and ZDF, have said they will stop reporting from Russia after its parliament passed a draft law punishing the publication of what it calls “fake news” about its invasion with jail terms of up to 15 years.
Award-winning independent Russian newspaper Novaya Gazeta says it will also stop reporting on the war in Ukraine in light of the new law, while several other Russian news outlets have suspended operations.