About 1,330 people were detained on Wednesday in spontaneous demonstrations in at least 38 Russian cities against the partial mobilisation announced by President Vladimir Putin, the independent civil rights portal OVD-Info reported on Wednesday evening.
Wednesday's demonstrations were the largest since the start of Russia's invasion of Ukraine in late February.
Agence France Presse journalists in Moscow saw at least 50 people arrested on one of the main streets of the capital. In Saint Petersburg, Russia's second largest city, a bus full of detainees was taken away by police from the city centre. Demonstrators chanted "No to war!" and "No to mobilisation!"
Russia's Interior Minister confirmed to Interfax that there had been a crackdown on "attempts to organise unauthorised demonstrations."
"Everyone was stopped and those who broke the law were arrested and taken to police stations," he said.
Earlier on Wednesday, Putin had announced a partial mobilisation of reservists. According to the Ministry of Defence, 300,000 people are to be called up.