At a long-planned opposition event Sunday that drew unexpectedly large crowds, demonstrators who gathered to protest Vladimir Putin's imminent inauguration as president clashed with riot police in the first major opposition protest to involve violence.

After months of largely peaceful protests against alleged vote fraud in recent parliamentary and presidential elections, Sunday's event seemed to signal a shift in atmosphere surrounding the nascent opposition movement.

A number of demonstrators were injured by riot police, who wielded batons in clearing crowds from Bolotnaya Ploshchad, the site of a planned opposition rally Sunday evening to protest Monday's presidential inauguration. Seventeen people requested medical care for injuries sustained during the event, a hospital source told Interfax. Around 450 protesters and opposition leaders Alexei Navalny and Sergei Udaltsov were arrested, police said.

And more than 20 cops were injured by protesters who threw pieces of asphalt broken off the street and bottles and hit officers with their fists, police said in a statement. Three of the injured police officers were hospitalized.

Despite the event's ambitious name, "March of Millions," organizers did not expect Sunday's event to draw the estimated tens of thousands who attended. City Hall had given advance approval for 5,000 participants to take part in the march and rally.

Police said 8,000 demonstrators passed through the metal detectors set up at the start of the march route, but people could reach Bolotnaya Ploshchad via an alternative route. Organizers estimated that there were as many as 100,000 protesters on hand. Udaltsov said on air to Kommersant-FM radio from the Yakimanka police station that organizers had not expected such a high turnout.

The event began similarly to an opposition march and rally held Feb. 4 on the same route, with participants gathering around 3 p.m. at Kaluzhskaya Ploshchad outside Oktyabrskaya metro station. Demonstrators who included parents with young children and pensioners passed through 14 metal detectors arrayed at the south end of Ulitsa Bolshaya Yakimanka.

Also on hand were opposition leaders Udaltsov, Navalny, Boris Nemtsov, and Yevgenia Chirikova, as well as contingents of nationalist activists and activists from liberal and leftist political groups, including the Solidarity movement, the Communist Party, and the Yabloko party.

But a shift in atmosphere was apparent in part from demonstrators' signs, which bore fewer creative messages than at past opposition events. One man held a sign with the direct message: "There's no 3rd term in our constitution," referring to controversial language in the constitution regarding the number of presidential terms allowed. A woman held a sign in the shape of a Russian Orthodox cross that said, in English, "Putin must die."

There also appeared to be a higher concentration of aggressive and radical demonstrators in the crowd than at previous opposition events.

Andrei Biryukov, 53, a computer programmer, said he attended the protest because he was against the current leadership.

As a taxpayer, "I hired Putin and Medvedev," he said, white ribbons-the symbol of the opposition-tied to both sides of his eyeglasses. "I want to throw them out and put in place normal people," he added.

Tension began to build once protesters arrived to the area surrounding Bolotnaya Ploshchad, where the rally was set to be held. Multiple lines of cops and OMON riot police had formed across the Bolshoy Kammeny Bridge leading toward the Kremlin and around the entrance to the square. A bottleneck formed at the entrance to Bolotnaya Ploshchad, and demonstrators feared a possible stampede, according to a Moscow Times reporter present in the crowd.

Navalny and Udaltsov called for a sit-in on the asphalt in front of a row of cops.

"We will not leave!" Udaltsov said, Interfax reported. "We have decided not to go anywhere; [we will] stand until victory." He said the demonstrators would present demands to authorities, including airtime on state-run television channels and new parliamentary and presidential elections, before they would leave the square.

At around 6 p.m., a group of demonstrators attempted to break through the line of police officers, who immediately began detaining people, in some cases violently.

Chirikova seemed to lay blame for the confrontation on the police, saying they were pressing the crowd.

"First, we sat on the asphalt. The police started to press people and beat them with truncheons," she said.

"They beat women and children and teenagers... . Of course the people started to resist," Chirikova told The Moscow Times.

Putin spokesman Dmitry Peskov said police acted properly and said they should have been even rougher with demonstrators, Kommersant-FM reported. Speaking to the radio station, Peskov said there was video evidence that demonstrators had provoked police and said police would have acted in the same way in Western countries.

The conflict escalated as demonstrators who appeared to be in their teens and twenties, some wearing masks over their faces, began throwing pieces of asphalt broken from the pavement and bottles filled with water at police, while the crowd yelled "Shame!" and "Fascists!" A Moscow Times reporter at the event saw one demonstrator throw a Molotov cocktail.

Demonstrators besieged a van of pro-Kremlin television channel NTV, with people putting stickers on the windshield, trying to pull off the windshield wipers, and hitting the van with their fists.

A van of state-run TV channel Rossia 24 was also vandalized, with papers and plastic bottles littered on the hood and windshield.

Udaltsov was detained by police a few minutes after taking the stage on Bolotnaya Ploshchad and speaking to demonstrators via multiple loudspeakers. Navalny was arrested soon after, while climbing the steps to the stage. Both will likely spend the night in jail and face possible 15-day jail sentences for not obeying police orders, Udaltsov's lawyer Violetta Volkova told Interfax.

From about 6:30 to 8 p.m., riot police dispersed the crowd in Bolotnaya Ploshchad by pressing cordons of officers and rushing at the demonstrators. In some cases, officers carried and dragged people away from areas they were clearing.

Police in boats moved along a canal next to the square fishing out riot helmets that had fallen into the water. Demonstrators also picked up some of the riot gear that had been stripped from officers.

"This is the first trophy for the museum of the revolution," said 31-year-old computer programmer Mikhail Zotov, holding up on a pole an OMON helmet he had retrieved from the river. Nearby protesters cheered as Zotov paraded the helmet around.

A Moscow Times reporter said most of the police officers did not act violently toward protesters but that some used excessive force.

One man, his shirt missing and red baton marks crisscrossing his back, was hoisted to his feet by officers. As they escorted him on both sides, another officer landed a blow across his bare back with a baton.

Officers hit another young man with batons until demonstrators approached, leaving the man sitting on the ground with his head bowed. An officer stepped away from him, then turned, took a step back and kicked him in the chest.

Several tents were set up by demonstrators, apparently in an attempt to camp out on the square. Opposition leaders have said they want to create a tent city as a way of applying pressure on authorities to meet their demands for political reforms.

Police herded all demonstrators out of Bolotnaya Ploshchad by around 8:30 p.m. Individual groups numbering several people each committed "provocations" against police on the nearby streets of Bolshaya Ordynka and Pyatnitskaya Ulitsa, a police spokesman told Interfax.

The Investigative Committee said it will conduct an inquiry concerning the reported attacks on police and alleged calls for riots on the part of organizers. The charge of violence against authorities carries a sentence of up to 10 years in prison, while someone convicted of calling for riots can be imprisoned for up to two years.

Across town at Poklonnaya Gora, a pro-Kremlin rally that police said was attended by 30,000 people took place without incident. Demonstrators waved flags of the state postal service and ruling party United Russia, and a series of musical groups performed on a stage, Kommersant-FM reported.

Opposition demonstrations of up to several hundred people were held all over Russia on Sunday, including in Vladivostok, Chelyabinsk, Kemerovo and Novosibirsk.