Last Updated May 5, 2018 8:58 AM EDT

MOSCOW — Russian police have detained opposition leader Alexei Navalny at a demonstration against President Vladimir Putin in central Moscow. Video showed police carrying a struggling Navalny, who is Putin’s most prominent foe, out of capital’s Pushkin Square, carrying him by the legs and arms. 

Thousands of demonstrators denouncing Putin’s upcoming inauguration into a fourth term gathered Saturday in the square for an unauthorized protest that was part of a day of nationwide demonstrations under the slogan “He is not our czar.” Activists from the pro-Kremlin National Liberation Movement were staging a rival rally at the same place Saturday, the BBC reports. A helicopter hovered overhead to monitor the crowd. 

There were protests in other cities, and a monitoring group said Russian authorities have arrested over 350 people in nationwide demonstrations against Putin.

Navalny was prevented in March from standing in the election. The election process in Russia is tightly managed by the government.

Putin will be inaugurated Monday for a six-year term.

© 2018 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.