OVD-Info said in a statement that 109 people had been detained in 23 cities as of 10.42pm in Moscow.
Authorities have clamped down heavily on signs of dissent since Russia's invasion of Ukraine in February 2022 and in most cities only a handful of people were held.
Navalny is serving combined sentences of 11 and a half years for fraud and contempt of court on charges that he said were trumped up to silence him.
Footage from Moscow and St Petersburg, Russia's two largest cities, showed police arresting individual demonstrators.
One man could be seen briefly holding up a sign before Moscow police ushered him away, bent over, as he groaned in pain.
Another man, who held up a sign in English that read "Free Navalny," was also arrested in Moscow.
In St Petersburg, a woman accompanied by a child told reporters that "I'm against the war, that's why they detained me with my underage kid".
Navalny, who rose to prominence by lampooning President Vladimir Putin's elite and alleging vast corruption, said in April that an "absurd" terrorism case had been opened against him that could result in him being sentenced to a further 30 years in jail.
