Russian opposition leader Alexei Navalny has been detained at home ahead of a planned unauthorised protest in Moscow, his wife said.
"Alexei has been arrested in the entrance to our block of flats," Yuliya Navalnaya wrote on Twitter, adding "our plans haven't changed". Thousands of his supporters have heeded his call to protest against corruption.
OVD-Info, an NGO, says 121 people have been detained in Moscow and 137 in St Petersburg. St Petersburg news website Fontanka.ru put the number of detainees there at 300.
There were smaller rallies in other Russian cities.