More than 700 people were arrested in Moscow and about 340 people in the second largest city Saint Petersburg, the monitor said.
A number of Russian activists reportedly called on social media for people to take to the streets after Putin launched the offensive on Ukraine in the early hours of Thursday.
One petition, started by a prominent human rights advocate, Lev Ponomaryov, garnered over 150,000 signatures within several hours and 289,000 by the end of the day, according to Al-Jazeera.
More than 250 reportedly journalists put their names on an open letter decrying the aggression. Another one was signed by some 250 scientists, while by 194 municipal council members in Moscow and other cities signed a third.
Earlier Thursday, Russian authorities warned anti-war sympathizers against gathering for protests.
The Investigative Committee, a government body that investigates major crimes, warned Russians of legal repercussions for joining unsanctioned protests related to “the tense foreign political situation”.
“One should be aware of the negative legal consequences of these actions in the form of prosecution up to criminal liability,” the Committee said.
Founded in December 2011, OVD-Info is an independent Russian human rights media project aimed at combating political persecution.
Source: Asia-Plus