The head of the local election coordinating committee and about a dozen activists have been detained by police in the Russian city of Nizhny Novgorod while attempting to hold a rally in support of opposition politician Aleksei Navalny.
Navalny, an anti-corruption crusader, has said he wants to stand for president next March, but the electoral authorities have said he is "not eligible to stand for office" because he is now serving a five-year suspended sentence for embezzlement.
Moscow police said Navalny was detained "over multiple calls to participate in an unauthorised public event".
Navalny said on social media on Friday that police had detained him in the lobby of his apartment block and told him they wanted to interview him at a police station.
As the opposition leader states, he was detained around his home.
The Tass news agency on Friday quoted police as saying that Navalny was detained because of his calls for unsanctioned rallies, which is punishable by up to 30 days in jail.
After he announced his presidential bid past year, Navalny, a top Kremlin foe and arguably Russia's most popular opposition politician, inspired a grassroots campaign in Russian regions to support his nomination.
The anti-corruption crusader linked his detention with another - bigger - rally scheduled in Saint Petersburg, Russia's second city and Putin's hometown, on Oct 7, the president's birthday.
After his release, Navalny tweeted that the authorities' efforts to derail opposition rallies will fail.
Russian Federation holds a presidential election in March which incumbent Vladimir Putin is widely expected to contest.
The city authorities said they had refused Navalny permission to hold the event.
That began to change earlier this year when Navalny opened campaign offices in 80 cities and towns, most of which had not seen a political life for decades, attracting thousands of supporters.