Russian opposition leader Alexei Navalny, one of President Vladimir Putin's biggest critics, was briefly detained by police on Sunday after a big campaign rally in his bid to become Moscow mayor.
Navalny, who led protests against Putin last year but is not expected to win the mayoral election on Sept. 8, put up no resistance as he was led to a waiting police van and taken away after addressing several thousand people outside a Moscow park.
Police said he was questioned about holding an unsanctioned meeting that "violated the rights and legal interests" of others but was quickly released. Navalny confirmed on Twitter that he had been freed and added: "All is well."
As police moved towards the stage where Navalny was speaking, the crowd had chanted: "Shame, Shame." But there were no clashes and he was able to finish the rally, although aides said several support staff had also been questioned by police.
One aide at the rally said authorities were putting pressure on Navalny in an attempt to disrupt his campaign to oust Moscow's pro-Kremlin mayor, Sergei Sobyanin.
Opinion polls show Navalny has little chance of defeating Sobyanin, but he wants to show that the opposition is capable of mounting an electoral challenge even though last year's anti-Putin protests have dwindled.
Navalny, 37, has a five-year prison sentence hanging over him, pending an appeal, after being found guilty last month of stealing timber from a state firm. He says the charges were invented as Putin's revenge for the protests.
A Moscow court last week rejected an attempt by Navalny to have Sobyanin disqualified on a technicality and an electoral official said Navalny himself might be thrown out for violating rules about the funding of campaign materials.
Although Navalny, an anti-corruption campaigner, is the leading opposition figure, Putin remains Russia's most popular politician, opinion polls show.
The Kremlin denies interfering in the mayoral campaign or in Navalny's trial and sentencing.