A Moscow court has ruled that Russian opposition leader Alexei Navalny should be jailed for 30 days for staging an unsanctioned rally in the city.
The anti-corruption rallies called for by Mr Navalny were held in more than 100 Russian towns and cities on Monday.
In Moscow, thousands of angry protesters held an unsanctioned rally on Tverskaya, the capital's main street.
More than 1,000 people have been arrested across Russia.
The judge at the Simonovsky district court ruled after midnight on Monday that Mr Navalny should be jailed for repeated violations of the law on public gatherings.
Mr Navalny was detained outside his home an hour before the rally was to start.
The Moscow protest was the most prominent in a string of more than 100 rallies in cities and towns stretching through all 11 of Russia's time zones - from the Pacific to the European enclave of Kaliningrad - with many denouncing President Vladimir Putin.
Thousands of angry demonstrators thronged to Tverskaya Street chanting "Down with the tsar" and singing the Russian national anthem.
The protests coincided with Russia Day, a national holiday that this year brought out historical re-enacters, some of them dressed in medieval costumes.
More than 700 people were arrested in Moscow, while in St Petersburg, about 500 were forced into police buses at an unsanctioned rally that drew up to 10,000 people.
The demonstrators appeared predominantly young - those who were born or grew up during Mr Putin's 17 years in power.
Similar crowds turned out on March 26, rattling officials who had perceived the younger generation as largely apolitical.
Three 16-year-old girls took sheets of paper to the Moscow protest and sat on the pavement to write the articles of the Russian Constitution on them; a nearby group of teenagers climbed on top of a tent with posters saying "Corruption kills the future".
Other protesters scaled a scaffold and hung a sign saying "Only revolution will defeat corruption".
Some of the rallies were sanctioned by authorities and peaceful, but police cracked down brutally on others.
Although it was not immediately clear if Monday's protests were larger than those in March, they underlined the deep dismay with the government.
Mr Putin is expected to seek another term in 2018, and Mr Navalny has already announced his intentions to run.
Moscow officials had agreed to allow Mr Navalny's rally, but late on Sunday, he said official interference had prevented contractors from erecting a stage at the agreed-upon venue and instead urged demonstrators to gather on Tverskaya Street, which was closed to traffic for the Russia Day festivities.
Most of the Moscow protesters appeared to be under 30, although there was a sizeable number of middle-aged people and couples with children.
With opposition sentiment strong or even growing, authorities appear to be seeking a strategy to undermine the opposition without provoking more animosity.
Mr Navalny rose to prominence for detailed open-source investigations of government corruption.
That was a key issue for protesters, particularly his report on vast wealth allegedly acquired by Prime Minister Dmitry Medvedev.
"We are against the corruption that is costing the future of our young people," said Moscow protester Maria Badyrova, a 33-year-old finance specialist who went to the rally with a Russian flag.
Alexei Borsenko, a Vladivostok demonstrator who eluded a police attempt to detain him, cited Iceland's prime minister stepping down in the fallout from the "Panama Papers" scandal, while "our prime minister is caught on such big corruption cases and he doesn't go anywhere".
"This is very strange," he added. "It's a dead end for the country's development."