Russian opposition leader Alexei Navalny was detained in central Moscow on Sunday amid an unauthorized rally of thousands of his supporters, part of the the biggest wave of protests the country has seen in years.
Moscow police estimated in the mid-afternoon that there were about 7,000 to 8,000 people marching through the city's central Tverskaya Street, where Navalny and at least 100 of his supporters were detained, Russian news agencies reported.
Navalny was detained "as soon as he appeared from the underground," a law enforcement source told Russian state news agency TASS.
Navalny quickly tweeted to let people know he was all right.
"Hey, everything is OK with me," he wrote. "Continue your peaceful walk. The weather is nice."
The opposition movement planned protests in 100 of Russia's largest cities Sunday, all designed to address complaints of entrenched corruption throughout the government.
The Moscow rally was the city's largest since the protest movements of 2011-13, which were fuelled by contentious elections for President Vladimir Putin, the parliament and the capital city's mayor.
Navalny, 40, came in second in Moscow's 2013 mayoral race and has announced intentions to run for president next year. However, a recently upheld conviction on corruption charges could preclude him from the ballot. His supporters have widely denounced the charges as trumped up.
A woman with a Russian flag draped across her shoulders said at the Moscow rally that she was walking in support of Navalny's efforts to expose corruption.
Navalny has only a small chance of becoming president, "but there is a chance," she told dpa.
Another protester yelled out to police officers who were running to cordon off a part of the central Pushkin Square: "You should serve the people," he said.
"We serve the president," an officer replied.
Russian authorities had warned Navalny and his supporters not to attend the rally because it had not been sanctioned by the city administration.
The marchers - many of them young - seemed to mostly draw police attention when their numbers spilled from the sidewalks into the streets. Police were out in large numbers in Moscow, many of them trying to keep Tverskaya Street, a main thoroughfare, open for cars.
Similar protests were reported throughout the country, including in the major cities of St Petersburg, Saratov, Novosibirsk, Krasnoyarsk and Vladivostok.
In Moscow, authorities had offered to let the marches proceed, but on streets that run along the city's outskirts. March organizers turned down that offer.
Moscow authorities had warned Navalny's followers that there would be consequences for going through with the march.
After the first marchers were detained, Navalny tweeted for his followers to keep on marching.
"Guys, everything is OK with me. You don't need to fight to free me. Walk around Tverskaya. Our theme for today is the fight against corruption."
Navalny released a video in early March in which he accused Prime Minister Dmitry Medvedev of gross corruption. He has also made his mark on Russian political discourse with regular releases of information alleging to prove official corruption.