Protesters use a banner to shelter themselves from rain, as they hold posters reading "The day of Russia without Putin", in Moscow today. Thousands of Russians are gathering today for the first massive protest against President Vladimir Putin's rule since his inauguration as investigators summoned several key opposition figures for questioning in an apparent bid to disrupt the rally. (AP Photo/Alexander Zemlianichenko)

LAURA MILLS and LYNN BERRY

MOSCOW (AP) - 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, as investigators sought to raise the heat on the opposition by summoning some of its leaders for questioning just an hour before the march.

The rally comes as a key test of Putin's line toward the opposition, following a quick passage of a repressive new bill introducing heavy penalties for taking part in unauthorized rallies.

A series of searches of opposition leaders' apartments and their interrogation were widely described as a crude attempt by the government to scare the protesters.

Leftist politician Sergei Udaltsov snubbed the summons, saying on Twitter that he considers it his duty to lead the protest as one of its organizers. The Investigative Committee said it wouldn't immediately seek his arrest and would interrogate him later.

Anti-corruption blogger Alexei Navaly, liberal activist Ilya Yashin and TV host Ksenia Sobchak attended the interrogation session that prevented them from attending the demonstration.

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

Sobchak, the only daughter of a late mayor of St.Petersburg, 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 Monday.

Braving a brief thunderstorm, protesters showed up on the landmark Pushkin Square ahead of the planned march and their number grew as they began marching down boulevards to a broad downtown avenue where the rally is to be held.

Despite fears of unrest following a violent police crackdown on a previous protest last month, the demonstration went on peacefully.

Udaltsov put the number of protesters at 50,000, while police estimated that about 18,000 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 that 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 investigators' action follows the quick passage last week of a new bill that will raise fines 150-fold on those who take part in unauthorized protests, to 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 an 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 has city approval, but any shift from the agreed upon location and timeframe could give police a pretext for a crackdown.

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.

The previous 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 the May 6 protest.