A Russian court on Tuesday sentenced opposition leader Alexei Navalny to 30 days in prison on charges of repeatedly violating law by organising illegal protests.
Earlier, baton-wielding riot police broke up anti-corruption protests and detained hundreds of demonstrators in Moscow and other Russian cities soon after arresting Navalny.
The protests, some of the biggest in Russia since 2012, were called by Navalny, a strong critic of Russian President Vladimir Putin.
"Russia without Putin" and "Russia will be free" chanted the demonstrators, including many young people, who crowded into central Moscow on a public holiday.
Navalny, who is mounting a long-shot bid to unseat Putin in an election next year, had called for mass protests in Moscow and other cities against official corruption.
The Kremlin has dismissed Navalny's graft allegations, accusing him of irresponsibly trying to whip up unrest.
TRT World's Julia Lyubova reports from Moscow.
Campaign maintains momentum
The scale of Monday's protests in Moscow and smaller ones in St. Petersburg and scores of other cities suggests Navalny has maintained his campaign's momentum despite more than 1,000 people being arrested after the last such protest in March.
That is likely to embolden him to call for more protests and keep Putin, who is expected to run for and win re-election next year, under rare domestic pressure.
"Neither mass detentions nor criminal cases after March 26 (the last protest) worked," wrote Lyubov Sobol, a Navalny ally, on social media. "People are not afraid."
The OVD-Info monitoring group, a non-profit organisation, said preliminary figures showed 730 people had been detained in Moscow.
The Interior Ministry said 500 people were detained in St Petersburg.
Navalny's wife, Yulia, said her husband had been detained as he tried to leave their home.
Navalny was accused of violating the law on organising public meetings and of disobeying a police officer, police said.
Authorities in Moscow said Monday's protest was illegal and drafted in riot police who fired pepper spray and used batons to break it up, detaining people and bundling them onto buses.
Moscow authorities had initially authorised a venue for Monday's protest away from the city centre.
But Navalny switched it to Tverskaya Street, Moscow's main avenue near the Kremlin. The General Prosecutor's Office had warned that a protest there would be illegal.
Roman, a 19-year-old student, said Navalny's campaign against official corruption had struck a chord.
"I'm sick of the Putin system," he said. "It's been unchanged for the last 17 years. There is so much evidence that our officials are stealing with impunity."
Dima, an 18-year-old florist, said he wanted Prime Minister Medvedev to return what he said were the politician's ill-gotten gains.
Medvedev, a close Putin ally, flatly denies wrongdoing.
The Interior Ministry said the turnout at the Moscow protest was about 4,500 - significantly fewer than the independent estimates which put the numbers in tens of thousands.
State media ignored the demonstrations, broadcasting Soviet-style coverage of Putin handing out state awards instead.
Anger over corruption
Navalny brought thousands onto the streets across Russia in March, the largest such protests since a wave of anti-Kremlin demonstrations in 2012.
Navalny was fined and jailed for 15 days for his role in those protests.
For now, polls suggest Navalny has scant chance of unseating Putin, who enjoys high ratings. It is unclear too if the Kremlin will even let Navalny run for the presidency.
But the 41-year-old lawyer turned political street campaigner hopes anger over corruption may boost his support.
A video he made accusing Medvedev of living far beyond his means has garnered over 22 million online views to date.
Navalny, who had a green liquid thrown in his face in April, robbing him of some of his sight, said hundreds of people had also attended demonstrations in Russia's Far East on Monday morning.
"I want changes," wrote Navalny in a blog post last week.
"I want to live in a modern democratic state and I want our taxes to be converted into roads, schools and hospitals, not into yachts, palaces and vineyards."
US condemns crackdown
The US condemned Russia's crackdown on anti-corruption protesters on Monday and called on Moscow to release peaceful demonstrators detained around the country.
"The United States strongly condemns the detention of hundreds of peaceful protesters throughout Russia," White House spokesman Sean Spicer said at a news briefing.
"Detaining peaceful protesters, human rights observers and journalists is an affront to core democratic values."