Updated at 6:30 p.m. ET
A Moscow court has sentenced Russian opposition leader Alexei Navalny to 30 days in jail, after police arrested him outside his home Monday ahead of nationwide demonstrations against Russian President Vladimir Putin.
Protesters turned out anyway, and security forces detained hundreds of demonstrators in both Moscow and St. Petersburg.
"Alexei was detained at the entrance of the house," his wife, Julia Bulk, wrote on Navalny's Twitter feed. She posted a photo showing police taking her husband to a police vehicle, captioning the photo, "Russia Day!"
A judge sentenced Navalny after midnight for "staging an unsanctioned rally," The Associated Press reported.
Other images from the day show thousands of demonstrators in St. Petersburg chanting slogans against Putin — and black-clad police marching through the crowd, making arrests and pulling people off of monuments.
NPR's Mary Louise Kelly was at the scene in Moscow:
"Under blue skies, security forces massed in Pushkin Square, a vast public space in central Moscow.
"Barricades went up, helicopters circled, lines of police in body armor and wielding batons pressed protesters...and the protesters surged back.
"Government security forces never appeared to lose the upper hand and by later afternoon here in Moscow, protesters had either been dispersed or hauled off to jail."
Some 500 people were detained in St. Petersburg, Interfax reports, citing police. In Moscow, more than 150 were detained in the city's downtown "for violating public order," says state-run Tass media.
The Interfax news agency adds that Navalny ran into problems setting up a planned demonstration on Sakharov Avenue, and instead told his allies on Sunday to gather on Tverskaya Street.
Navalny, who was also arrested for organizing large anti-Putin protests in March, had called for national demonstrations on the holiday that marks Russia's formal emergence from the Soviet era in 1990. He's hoping to challenge Putin in next year's presidential election, delivering speeches around the country, as NPR's Lucian Kim reported last week.
Mary Louise reported earlier in the day that while the crowd in Puskin Square was huge, it's also difficult to ascertain how many people are there for Navalny's cause, and how many for the national holiday.
"There are people walking around the streets in World War I uniforms," Mary Louise says. "There are big stages set up for concerts."
She adds that while the mood in the square had seemed peaceful earlier Monday, "the situation is changing very quickly" with "images streaming in of police and protesters pushing toward one another" as bystanders struggle to get out of the way.
The arrests began, Mary Louise says, after the crowd began chanting — something the authorities had warned them not to do.
From images that we're seeing, many young people were among the protesters — and many were detained. After opposition activist Ilya Yashin, 33, was detained and put in a police bus, he tweeted an image from inside, showing himself with a half-dozen youths in Moscow.
And in St. Petersburg, several teenage girls were arrested, as Financial Times journalist Max Seddon tweeted.
"I am glad that the protest movement is getting younger," Yashin said in a posting that showed a young woman next to riot police late Monday. It was an answer, he added, to those who complain Russia has no future.
An unlikely object of dispute emerged at the St. Petersburg demonstration: As Russian journalist Arseny Vesnin and others noted, a giant yellow duck was among those detained by police, after being batted around in the air by the crowd.
By making an appearance, the duck was reprising its role in the large March protests. It's a reference to a report by Navalny's Anti-Corruption Foundation from last fall, which said that a vacation estate frequented by Prime Minister Dmitry Medvedev has a special house just for ducks.