Moscow - Tens of thousands of protesters chanting "Russia Will be Free" rallied in Moscow on Tuesday against President Vladimir Putin's third term despite a police crackdown on their leaders a day earlier.
The largely youthful crowds, many wearing the movement's symbolic white ribbons, made their way down leafy boulevards from Moscow's central Pushkin Square as Putin warned in a speech to mark Russia's national holiday that any upheaval would not be tolerated.
One event organiser put attendance at the so-called March of Millions at more than 100 000 while the city's police department counted just 18 000 participants.
However aerial television footage of the main rally suggested a figure of at least double that quoted by the police.
"Dear friends, they are afraid of us and we aren't afraid of them," veteran protest leader Boris Nemtsov told a sea of people on Moscow's broad Sakharov Avenue - the scene of similar demonstrations more than six months ago.
"We live in occupation and we are fighting occupiers on our territory," added prominent environmentalist and blogger Yevgenia Chirikova to cheers of approval.
The protest came just a day after the homes of its leaders were raided in a police crackdown that was condemned by Washington and sparked concerns of a new bid by the former KGB agent to turn back the clock on past reforms.
Almost all the main leaders of the social network-driven protest were absent because they were being questioned by investigators, who denied hauling them in for the sole purpose of disrupting the march.
But Putin vowed on Tuesday not to let Russia be shaken by social upheaval during his six-year term.
"We cannot accept anything that weakens our country or divides society," Putin said in a televised address commemorating the Russia Day national holiday.
"We cannot tolerate decisions and actions capable of leading to social and economic shocks."
Analysts view the nascent protest movement as the biggest challenge that has confronted the strongman leader during his 12-year domination of Russia as both president and more recently as premier.
US expresses concern
Putin has preferred to treat the protests dismissively and has repeatedly accused the US State Department of helping incite the demonstrations through indirect funding.
State Department spokesperson Victoria Nuland told reporters on Monday that Washington "is deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations".
But police officers appeared on the Sakharov Avenue stage to present Nemtsov and fellow speaker Sergei Udaltsov warrants to appear for questioning at the powerful Investigative Committee later in the day.
The two are being probed along with the anti-corruption blogger Alexei Navalny and television presenter Ksenia Sobchak and others over their roles in bloody clashes at a protest held on the eve of Putin's inauguration.
"It is terrible to be sitting here when things on Sakharov are so fun and cool," Navalny tweeted in the course of his questioning.
"They are asking about our incomes going back to January 1, 2005."
He and the others were still being interrogated more than four hours after the initial march's start.
The websites of Russia's main independent news sources meanwhile became inaccessible just as the protesters began gathering at the central Puskin Square for their march.
Those affected included the Moscow Echo radio station and Dozhd (Rain) TV website as well as prominent opposition Novaya Gazeta twice-weekly newspaper.
The websites of Russia's main media sources - including Kremlin-allied papers and state-controlled television stations - were all accessible and operating without delay.
The three opposition media switched their news feeds to the main social networks popular with Russians and blamed the breakdown on a distributed denial-of-service (DDoS) attack of unknown origin.
Opposition leaders accuse pro-Putin youth groups of staging such attacks with the Kremlin's tacit approval at sensitive political times.