MOSCOW — As thousands of riot police officers flooded central Moscow on Saturday to curb protests calling for fair elections, the Russian authorities announced they had opened a criminal investigation into money laundering against an anticorruption organization led by Aleksei A. Navalny, Russia’s most prominent opposition activist.
The case against his organization, the Anti-Corruption Foundation, was opened by the Investigative Committee, Russia’s version of the F.B.I., the state news agency Tass reported. It involved funding for the anticorruption group’s work of 1 billion rubles (around $15 million) in “money obtained by criminal means.”
The money-laundering case is a sharp escalation in the Kremlin’s drive to silence Mr. Navalny, the driving force behind a surge of public dissent in recent weeks, and to snuff out opposition to President Vladimir V. Putin, whose popularity has slumped as Russia’s economy continues to stagnate.
Fearful that even modest peaceful protests could snowball into a serious challenge to Mr. Putin and his so-far secret plans for what will happen when his supposedly final term ends in 2024, the authorities have taken an increasingly hard line against all forms of dissent on the street.
Mr. Navalny was arrested and sentenced to 30 days in jail last month for organizing an authorized protest after the authorities barred several opposition candidates from running for Moscow’s City Council. Officials claimed that the candidates had falsified signatures on petitions to run, a charge the opposition candidates denied.
A day after an unauthorized election protest in Moscow on July 27, Mr. Navalny was hospitalized with what officials called a “severe allergic reaction” in jail. But his regular physician, Anastasy Vasilyeva, attributed his swollen face and burned eyes to possible poisoning with a “chemical substance.”
On Saturday, a police officer grabbed Lyubov Sobol, another prominent opposition figure, as she took a taxi to a protest along the Boulevard Ring, a tree-lined road and pedestrian walkway in the center of Moscow.
OVD-Info, an independent group that monitors arrests, reported that nearly 600 people had been detained by Saturday afternoon. (Last weekend’s protests drew nearly 1,400 arrests.) Most are likely to be released at the end of the day, but some of those detained in earlier protests have been charged with rioting, a crime punishable by up to 15 years in jail.
The huge display of police muscle on Saturday seems to have deterred many people from joining the second weekend protest march, which had been called by Mr. Navalny’s organization and other opposition groups to denounce the exclusion of the dissident candidates from the September election.
After years of denouncing Mr. Navalny and like-minded Russians as a “nonsystem opposition” bent on overturning Russia’s established order, the Kremlin has prompted fury by making it impossible for them to enter the political system.
Konstantin Yankauskas, a would-be candidate barred from taking part in the September elections, was released from prison on Saturday — and then bundled into a police van waiting for him outside the detention center.
The crowds were far smaller than the July 27 rally outside Moscow City Hall. Instead of congregating in one place, Saturday’s protesters were scattered along an inner ring road. A group of protesters clapped rhythmically as they walked past a statute of Vladimir Vysotsky, a late-Soviet-era singer and counterculture icon.
A man walking past a line of police officers said to them, “You could be in Siberia fighting fires, instead you’re waving nightsticks.” It was an apparent reference to the 7.4 million acres of forest ablaze in Siberia.
Many of those who joined the protest were young Russians who have known nothing but the nearly 20-year rule of Mr. Putin. But many older Russians joined in. Galina Georgievna, a retiree who lives near Pushkin Square, said she came out because of “the thievery, the corruption; people have had it up to here.”
“I feel sorry for young people. I’m already an elderly person, but I feel sorry for them,” she said, adding that “if you leave Moscow, I have no idea how people survive. There are fires in one place, floods in another. Pensions are small.”
As with last weekend’s protest, which the authorities condemned as a “mass disturbance,” Saturday’s march was peaceful, with violence coming only from police officers.
“We came to defend our rights. We want the candidates to be allowed. You see what is happening in Russia,” said Denis Malygin, 30, an underwriter at a Moscow bank.
He and his girlfriend, Viktoria Vasilyeva, 24, have been attending the recent demonstrations to show their anger at the heavy-handed intolerance of dissent shown by the authorities.
“I experienced it myself,” he said. “I was simply an observer last week. I was filming all of the abuse of power, and I also had my arms roughly pinned to my sides and was put into a paddy wagon.
“Twenty other people and I were delivered to the Nizhegorodsky Police Department,” he added, “where we were kept and released only late at night.”
In Trubnaya Square near the Moscow Circus, a young man in a suit stood silently with a sign reading, “Give Us Back Our Elections,” as riot police officers — called “cosmonauts” because of their helmets and body armor — swept through the esplanade in a pincher operation, trapping sightseers, circus-goers and journalists.
The Moscow city prosecutor’s office warned on Friday that law enforcement agencies would “take all necessary measures to stop provocations, riots and any actions entailing a violation of public security.”
In Pushkin Square, the police broadcast a message warning protesters gathered around a statute of the revered Russian writer Alexander Pushkin that they were breaking the law and would “held responsible” if they did not leave.
The Russian Constitution guarantees the right to public protest, but a law requiring that all protests by more than one person obtain official permission has largely invalidated this right and has allowed the authorities to classify peaceful gatherings as criminal actions.