Russian police on Sunday arrested more than 100 people who had taken to the streets to mark the 47th birthday of Alexei Navalny, Russia's most prominent opposition leader, a protest monitoring group said.

OVD-Info said in a statement that 109 people had been detained in 23 cities as of 10:42 p.m. Moscow time (1942 GMT). 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 11-1/2 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 see him sentenced to a further 30 years in jail.

(Reporting by Reuters, writing by David Ljunggren; editing by Grant McCool)