Dozens of people were arrested at a protest in Moscow on Wednesday against constitutional reforms that give President Vladimir Putin the option to remain in power for another 16 years, witnesses and a monitoring group said.

About 500 demonstrators, many of whom wore face masks branded with the word "no," chanted calls for Putin to resign and held up banners against the reforms.

Police surrounded them and began making arrests late in the evening after participants started a march down one of the city's main boulevards, with officers in riot gear forcefully rounding up protesters and placing them in vans.