MOSCOW » Tens of thousands of Russians flooded Moscow’s tree-lined boulevards Tuesday in the first massive protest against President Vladimir Putin’s rule since his inauguration in May — a rally that came even as police interrogated key opposition leaders.

Since embarking on his third presidential term, Putin has taken a stern stance toward the opposition, including signing a repressive new bill last week introducing heavy penalties for taking part in unauthorized rallies.

Police on Monday searched opposition leaders’ apartments, carting away computers, cellphones and other personal items. They also demanded that opposition leaders come in for questioning Tuesday just an hour before the rally began — widely seen as a crude attempt by the government to scare the protesters.

The march was being held on Russia Day, a national holiday that honors June 12, 1990, when Russian lawmakers decided that Russian laws should take priority over Soviet Union laws. The Soviet Union then collapsed in 1991.

Leftist politician Sergei Udaltsov snubbed the summons, saying he considered it his duty to lead the protest as one of its organizers. Russia’s Investigative Committee said it wouldn’t immediately seek his arrest but would interrogate him later.

Udaltsov said he and another opposition leader, Boris Nemtsov, were handed summons by police right at the rally.

Anti-corruption blogger Alexei Navaly, liberal activist Ilya Yashin and TV host Ksenia Sobchak showed up for the interrogations that prevented them from attending the demonstration.

"It’s horrible to sit here while you are having fun," Navalny tweeted from the Investigative Committee headquarters.

Investigative Committee spokesman Vladimir Markin said authorities had found more than €1 million ($1.25 million) and $480,000 in cash at Sobchak’s apartment and would initiate a check to see whether she had paid her taxes.

Sobchak, a glamorous socialite described by some as Russia’s equivalent of Paris Hilton, insisted that she had done nothing wrong and was keeping her savings at home because she doesn’t trust banks. The authorities are likely to use the piles of cash to paint the opposition as a bunch of spoiled rich kids at odds with the majority of Russia’s population.

Sobchak, the only daughter of St. Petersburg’s late mayor, a man who was Putin’s mentor, had been spared reprisals until Monday’s raid. "I never thought that we would slide back to such repressions," she tweeted.

Braving a brief thunderstorm, protesters showed up on the landmark Pushkin Square ahead of the planned march and their numbers grew as they began marching down boulevards to a broad downtown avenue where a rally was being held. Despite fears following a violent police crackdown on a previous protest last month, the demonstration went on peacefully.

Speaking at the rally, Udaltsov reaffirmed a call for early presidential and parliamentary elections. He put the number of protesters at 100,000, while police estimated that about 20,000 showed up.

"Those in power should feel this pressure. We will protest by any means, whether peacefully or not," said Anton Maryasov, a 25-year-old postgraduate student. "If they ignore us, that would mean that bloodshed is inevitable."

Another protester, 20-year-old statistics student Anatoly Ivanyukov, said attempts by authorities to disrupt the rally would only fuel more protest. "It’s like when you forbid children to do something, it makes them even more willing to do that," he said.

The police investigators’ action follows the quick passage last week of a new bill that raises fines 150-fold on those who take part in unauthorized protests — fines that are nearly the average annual salary in Russia.

"I can’t predict whether I’ll leave here freely or in handcuffs," Yashin told reporters before entering the Investigative Committee headquarters for the interrogation. "The government is doing everything possible so that I don’t end up there (at the protest)."

The top Twitter hashtag in Russia on Monday was "Welcome to the Year ’37," a reference to the height of the purges under Soviet dictator Josef Stalin.

Tuesday’s protest had city approval, but any shift from the location and time could give police a pretext for a crackdown.

Udaltsov urged protesters to march across town after the rally to the Investigative Committee’s headquarters to demand the release of political prisoners — an action that would likely trigger a harsh police response.

Many in the crowd, however, seemed reluctant to risk defying the authorities. Alexei Moiseyev, a student, said he wouldn’t risk attending Tuesday’s rally if it was unsanctioned.

"I’m not ready to enter into conflict with the law, even if these laws are questionable," he said.

Opposition leader Nemtsov, speaking after Udaltsov, urged the demonstrators to act within the law. "We must act in a responsible way, peacefully and calmly," he said.

Sergei Parkhomenko, a leading journalist who helped organize Tuesday’s protest, said the authorities would like to see unrest to back their criticism of the opposition.

"They would be happy to stage some kind of provocation to prove that the people are just a herd of animals and the animals are always out of control," he said.

A big opposition rally a day before Putin’s inauguration in May ended in fierce clashes between police and protesters, and some opposition activists said the violence was provoked by pro-Kremlin thugs. The raids of the opposition leaders’ homes and their questioning were connected to that May 6 protest.

Andrey Bulay in Moscow contributed to this report.