Russia's opposition leader, Alexei Navalny, said he had been detained by police weeks ahead of a presidential election in which he has been barred from running.
Mr Navalny, who is Vladimir Putin's most visible and vehement foe, said on his Twitter account he was detained after visiting the dentist.
"They are taking me somewhere," he said, according to The Independent.
The anti-corruption campaigner has been repeatedly jailed for organising unauthorised protest rallies throughout the country.
He aimed to run for president in next month's election, but was denied a place on the ballot because of a fraud conviction many view as politically motivated.
Mr Navalny was released by police less than an hour after he was detained in Moscow.
He tweeted to say he had been released and given a document on legal procedures against him for organising illegal protests.
"They offered me a lift somewhere, but I declined and have gone to work," Mr Navalny tweeted. "I don't understand what happened, and why it took seven people to detain me."
If found guilty, he faces up to 30 days in jail.
Mr Navalny, a 41-year-old anti-corruption campaigner who organised some of Russia's biggest protests of recent years, says he has been unfairly barred from contesting the 18 March presidential election over what he says is a trumped-up suspended prison sentence.
Mr Putin is widely expected to win the contest, but Mr Navalny has called for a nationwide boycott aimed at undermining the President's legitimacy by bringing the turnout down.
Mr Navalny's detention came shortly after his campaign chief, Leonid Volkov, also said on Twitter he had been detained by police at Moscow's Sheremetyevo airport.
Tweeting before his detention, Mr Navalny said Mr Volkov had been planning to fly to the Russian regional republic of Bashkortostan to organise election-monitoring activities.