Floods began when a dam overflowed following heavy rains, uprooting thousands of people
Some 75,000 fans flocked to Wednesday's event in a Moscow stadium which came the day after frontman Bono held talks with rock-loving President Dmitry Medvedev on issues including preventing the spread of polio and HIV.
Bono praised Medvedev as "gracious" in front of the crowd but also as a finale invited Russian rock star Yury Shevchuk — famous for his outbursts against the Kremlin — to the stage for a duet.
Police not only forced out activists handing out leaflets and gathering signatures but also U2's own charity fund, the ONE Campaign against AIDS, activists said on Thursday.
"The tents of Amnesty International, Greenpeace and the ONE foundation were removed by police and we were not allowed to collect signatures and to talk to people," Greenpeace Russian director Ivan Blokov said.
"Our activities were agreed with U2's management, so we are very much surprised," he told AFP.
A Moscow police spokesman was quoted as saying activists had provoked the detentions by holding an unsanctioned protest in the Russian capital, though activists protested they had been present at each of U2's European concerts.
"All of that held the unquestionable trappings of an unsanctioned picket," the Interfax news agency quoted an unnamed police spokesman as saying.
Amnesty International's Russia head told AFP that five of the rights watchdog's activists were detained ahead of Wednesday's concert.
"It is sad that in Russia — which is considered a civilised country — the collection of petition signature so worries the authorities," Sergei Nikitin said.
"You get the impression that the authorities are afraid of their own citizens."
He protested that Amnesty had carried out similar awareness work with U2's encouragement throughout the band's European tour and that two of its activists had in fact travelled with U2 from the United States.
"I don't know if Bono knows about what happened to us," he said, adding that the group's leader was one of the organisation's chief activists.
"It was a typical publicity event, which this organisation has carried out in every city where U2 has preformed," he said.
For the show's finale, Bono invited Russian rock star turned anti-Kremlin activist Shevchuk onstage for a rendition of "Knocking on Heaven's Door", hailing the veteran singer as a "great man."
"What a time we've had in this extraordinary city of yours," Bono was quoted on U2's website as saying during the concert. "An amazing singer, Yury Shevchuk, is with us tonight. What a great man!"
Shevchuk on Sunday had appeared in front of some 2,000 people for a banned concert in central Moscow protesting plans to build a motorway through a forest where he was forced by the security forces to sing without any amplification.
Leading rights campaigner and Kremlin critic, Lev Ponomaryov, was jailed for three days by a Russian court Wednesday for taking part in the demonstration.
The 68-year-old activists' daughter told Interfax on Thursday that doctors were worried over Ponomaryov health in detention as the veteran campaigner suffered from high blood pressure.
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