A political rights group says Russian police detained at least 59 demonstrators in central Moscow on April 2.
The arrests were made at several locations after demonstrators gathered following calls on the Internet for protests. It was unclear who was behind those calls.
They come a week after more than 1,000 people were arrested during a demonstration organized by leading Kremlin critic and anticorruption activist Aleksei Navalny.
It was also unclear how many of the protesters detained on April 2 were government opponents. The website OVD-Info, which tracks police actions against protesters, said that one man who was detained carried a Russian flag and shouted "Putin is No. 1!"
In a separate incident, activist Ildar Dadin was arrested as he picketed alone outside an Interior Ministry building in Moscow, his lawyer Ksenia Kostromina said.
Dadin and many other detainees were later released.
Dadin was convicted and sentenced to prison in December 2015 under a controversial law that criminalizes participation in more than one unsanctioned protest in a 180-day period.
He was freed on February 26 after the Supreme Court threw out his conviction and ordered the case closed.
Dadin, the only person in Russia who has been convicted under the law, was considered a political prisoner by major human rights groups.
Navalny, who is currently serving a 15-day jail term for disobeying police at last week's rally, has distanced himself from the latest protest.
"He knows absolutely nothing about this," his spokeswoman Kira Yarmysh wrote on Twitter.
The March 26 protests, which brought tens of thousands of people into the streets in dozens of Russian cities, were the largest antigovernment protests in the country in the last five years.
The Russian government said on April 1 that it had opened a criminal investigation against the unidentified people who had called for the April 2 unsanctioned anticorruption demonstration in the capital.
The government had reportedly blocked access to a number of Internet pages calling for the protests.
