At least two Amnesty International volunteers were arrested while handing out flyers at a U2 gig in Moscow on Wednesday night, Ekho Moskvy radio station said.
During the concert, Russian rock veteran and well-known Kremlin critic Yuri Shevchuk was invited on stage to join the Dublin four-piece in a rendition of Bob Dylan classic "Knocking On Heaven's Door."
Shevchuk, who famously spoke out against Russian Prime Minister Putin in May, attracted media attention on Sunday when he played on stage in front of some 2,000 people at an environmental protest rally in central Moscow.
Police also closed information desks set up by Amnesty International, Greenpeace Russia and the ONE Campaign, an international aid organization, despite an earlier agreement with the U2 management.
Police said the volunteers staged an "unsanctioned" rally - a term Russian officials have come to abuse recently in increasing attempts to clamp down on protesters.
Greenpeace Russia said U2's management was unsure how to react to the police activity, since this is the first time such a thing has happened at one of the band's gigs.
On Tuesday, U2 lead singer Bono met with Russian President Dmitry Medvedev in the Black Sea resort city of Sochi. Besides music they discussed various social and charity activities that Bono and his musicians are involved in.
MOSCOW, August 26 (RIA Novosti)
