Rights group Amnesty said five of its activists were detained while distributing flyers at a U2 concert in Moscow on Wednesday, which ended with a celebrated Kremlin critic joining the Irish rock stars on stage.

Police detained the five volunteers who were distributing leaflets and displaying banners for holding an unsanctioned protest at Moscow's Luzhniki stadium, the head of Amnesty International in Russia Sergei Nikitin told the Interfax news agency.

I am very sorry about what happened ... it overshadowed the concert, Nikitin said. He said the activists were later released without charge. Interfax quoted an unnamed police official as saying two activists were detained.

At the end of their concert, U2 invited Russian rock star turned Kremlin critic Yury Shevchuk to join them on stage for a rendition of Knocking on Heaven's Door.

Shevchuk has become a significant figure in Russia's opposition movement since he delivered a rare public rebuke to Prime Minister Vladimir Putin in May.

Shevchuk's appearance with a guitar at an opposition protest at the weekend attracted 2,000 people, making it one of Moscow's biggest protests in years.

U2 frontman Bono met Russian President Dmitry Medvedev on Tuesday to try to convince him to support his efforts to combat AIDS. He did not make any public criticism of the Russian leadership during his trip.

A spokeswoman for U2 said the band did not yet have the details of the detentions and could not immediately comment. An Amnesty spokesman said he could not immediately comment.

(Writing by Conor Humphries; Editing by Jon Hemming)