Russian opposition leader Alexei Navalny was arrested outside his Moscow home as thousands of anti-government activists challenging President Vladimir Putin's rule protested across the country.
Mr Navalny's wife, Yulia, said on his Twitter feed that he was arrested about half an hour before the main demonstration in Moscow was to begin. Police later confirmed the arrest, saying he could get up to 15 days in jail on charges of failing to follow police orders and violating public order.
Although city authorities had agreed to a location for the Moscow protest, Mr Navalny called for it to be moved to Tverskaya Street, one of the city's main thoroughfares. He said contractors hired to build a stage at the agreed-upon venue could not do their work after apparently coming under official pressure.
Tverskaya, known in Soviet times as Gorky Street, was closed off to traffic for an extensive commemoration of the national holiday Russia Day, including people dressed in historical Russian costumes.
Moscow police blocked part of the street with trucks in a bid to block the movement of protesters. Associated Press journalists witnessed at least 50 arrests in St Petersburg, Russia's second-largest city, with Russian media reporting at least 150 arrested there. AP reporters also saw at least 10 arrests in Moscow, with reports saying there were up to 100 so far in the capital.
After the change, Moscow police warned that "any provocative actions from the protesters' side will be considered a threat to public order and will be immediately suppressed".
С Днём России! pic.twitter.com/CkfZXT6EKo— Alexey Navalny (@navalny) June 12, 2017
The protesters, some carrying Russian flags, were chanting loudly as Moscow riot police stood watch.
A regional security official, Vladimir Chernikov, told Ekho Moskvy radio that police would not interfere with demonstrators on the street - as long as they did not carry placards or shout slogans.
More than 1,000 protesters were arrested at a similar rally on March 26.
The protests in March took place in scores of cities across the country, the largest show of discontent in years and a challenge to Mr Putin's dominance of the country.
The Kremlin has long sought to cast the opposition as a phenomenon of a privileged, Westernised urban elite out of touch with people in Russia's far-flung regions. But today's protests could demonstrate that it has significant support throughout the vast country.
Mr Navalny's website reported that protests were held in more than a half-dozen cities in the Far East, including the major Pacific ports of Vladivostok and Khabarovsk and in Siberia's Barnaul.
Eleven demonstrators were arrested in Vladivostok, according to OVD-Info, a website that monitors political repressions.
Mr Navalny has become the most prominent figure in an opposition that has been troubled by factional disputes. He focuses on corruption issues and has attracted a wide following through savvy use of internet video. His report on alleged corruption connected to Prime Minister Dmitry Medvedev was the focus of the March protests.
Mr Navalny has announced his candidacy for the presidential election in 2018. He was jailed for 15 days after the March protests. In April, he suffered damage to one eye after an attacker doused his face with a green antiseptic liquid.