Police interrogated several leading opposition figures Tuesday after surprise raids at their homes the day before.
The interrogations forced the activists to miss a major anti-Kremlin protest and suggested that authorities were stepping up efforts to stifle dissent.
Alexei Navalny, Ilya Yashin and Ksenia Sobchak spent much of the day answering questions about documents and large sums of money seized in Monday's raids, as well as about clashes between police and protesters at a rally May 6.
Opposition leader Sergei Udaltsov was also ordered to appear before the Investigative Committee but chose to go to the protest instead, telling investigators he felt it was his responsibility to lead the rally he played a key role in organizing.
Officials agreed to question Udaltsov after the rally.
The raids early Monday morning cast an added chill over political circles. They came three days after President Vladimir Putin signed a law drastically raising fines on illegal protests and fueled suspicions that the government was moving to defang the opposition.
The reaction on the Internet was explosive. A Russian Twitter hashtag meaning "Hello, 1937" — a reference to Stalin's purges of 1937 — quickly became a worldwide trend.
The crackdown was also met with unease abroad.
The European Union's high representative for foreign affairs, Catherine Ashton, expressed worry "about the steps taken recently in Russia to limit the scope for public rallies," according to a statement.
U.S. State Department spokeswoman Victoria Nuland said the U.S. was "deeply concerned" about the targeting of opposition leaders.
"These measures raise serious questions about the arbitrary use of law enforcement to stifle free speech and free assembly," she said during a press briefing in Washington on Monday.
The opposition leaders appeared for questioning with their lawyers just an hour before the protest was set to begin, making it nearly impossible for them to attend.
Sobchak was released at 4:15 p.m. and Yashin about 5 p.m., not long before the rally at Prospekt Akademika Sakharova was set to end.
The committee finished questioning Navalny at 5:20 p.m., but investigators then took him to the office of his Fund for Fighting Corruption to conduct a search, he
In a statement, the Investigative Committee
"The investigation will clarify whether taxes have been paid on the sum," the statement read. "The investigation will also seek to determine what purposes such a large amount of money was to be used for."
The statement said the investigation was wide-ranging and would continue.
Navalny said he was questioned about his activities dating back to 2005, including specific questions about Gunvor, an oil-trading company with ties to the Kremlin that he has accused of money laundering.
Yashin said he was peppered with questions about where the money financing the opposition was coming from and about the participants of the May 6 protest. He said he invoked his constitutional right not to answer.
"My main feeling after the interrogation was that investigators have been given the task of proving that Navalny, Udaltsov and myself masterminded mass disorder," he wrote on Twitter, adding that he had been called for more questioning Friday.
Sobchak complained that authorities had confiscated her passport.
"This means that I am effectively banned from traveling although I have not been charged with anything," she wrote on Twitter.
Also questioned Tuesday were Udaltsov's wife, Anastasia, and opposition organizers Maria Baronova, Alexei Sakhnin and Maria Dobrokhotova.
Monday's raids also targeted apartments belonging Baronova, an aide to State Duma Deputy Ilya Ponomaryov; Sakhnin, the creator of Navalny's Rospil anti-corruption organization; Navalny's in-laws and Udaltsov's parents.
Investigators also raided the offices of Rospil on Monday, while a search of veteran opposition leader Boris Nemtsov's apartment was called off because he wasn't home.
At Tuesday's rally, Nemtsov was handed a summons to come in for questioning, which he did following the event.
About 400 people were detained after clashes between police and protesters during the May 6 rally.
One man, Stepan Zimin, remains in custody. Six others have been detained in the weeks since the event, but only three have been formally charged.
Activist Mikhail Maglov was detained Monday and was questioned for eight hours about the May 6 rally and then released, said Solidarity movement spokeswoman Olga Shorina. Investigators confiscated his cell phone and laptop, she said.
Some observers said the Kremlin was seeking to divide the opposition by removing openly hostile elements from the leadership.
"There are Kremlin-friendly elements within the opposition. The Kremlin wants to make sure that they're the ones running the show," said Mark Feigin, a senior member of Solidarity.
Others said the hard-line approach would likely fail and serve only to fuel continuing protests.
"What's happening is a very unwise tactic by the Kremlin," said independent political analyst Stanislav Belkovsky. "These mosquito bites will only make the ones being bitten even more angry."