OVD-info, which monitors arrests during protests, said police had detained at least 5,016 people in 60 cities during the anti-war protests.
That is an unprecedented number for a single day and far higher than arrests made during a wave of protests that broke out in the country last year when opposition leader Alexei Navalny was imprisoned.
Navalny has called on Russians to protest the war in an appeal from his prison cell.
Protesters risk possible prison sentences by taking to the streets.
OVD-info said 2,394 had been arrested in Moscow.
Saint Petersburg saw at least 1,253 arrests, the monitor said.
Protests also took place in small cities in the Russian regions.
Several activists posted videos of violent arrests on social media.
OVD-info said police had used electric shockers on protesters.
Anti-war protests in Russia have continued despite warnings from authorities and risks of prison sentences.
To further stifle criticism, Putin on Friday signed into law a bill that introduces jail terms of up to 15 years for fake news about the Russian army.
The bill stipulates jail terms of varying lengths and fines against people who publish “knowingly false information” about the military, with harsher penalties to hit when dissemination is deemed to have serious consequences.
Putin also signed a bill that would allow fines or jail terms of up to three years for calling for sanctions against Russia with Moscow facing harsh economic penalties from Western capitals over the invasion.
