MOSCOW (AP) _ Popular anger against Vladimir Putin's ruling party and alleged election fraud have boiled over into a third straight night of protests, and police in Russia's two largest cities arrested scores of demonstrators.

The demonstrations in Moscow and St. Petersburg on Wednesday appeared to attract fewer protesters than in previous days, roughly 300 in each city, but Russians' willingness to risk jail time and clashes with police indicated significant tensions that could spread.

More than 17,000 people have signed up for a Facebook page calling for a massive demonstration Saturday on Moscow's Revolution Square. Authorities have sanctioned the rally, but say it has to be limited to 300 participants, so a far larger turnout would almost certainly provoke a harsh police response.

Putin's United Russia party lost a significant share of its seats in Sunday's parliamentary election for the State Duma, the lower house of parliament, but will still have a majority. Opponents say even that result was achieved by widespread vote fraud.

United Russia had two-thirds of the seats in the 2007 vote, making the party unassailable and allowing it to push through constitutional changes.

The latest protests came hours after former Soviet leader Mikhail Gorbachev urged Russian authorities to annul the results of Sunday's vote and hold a new one, according to the Interfax news agency.

``More and more people are starting to believe that the election results are not fair,'' he told Interfax. ``I believe that ignoring public opinion discredits the authorities and destabilizes the situation.''

Gorbachev, whose 1985-91 rule ended with the collapse of the Soviet Union, remains widely admired abroad but is widely disliked or regarded as insignificant at home. His call for a new election could further encourage the opposition, but is unlikely to influence those in power.

The 80-year-old Gorbachev has long had tense relations with Putin, but until recent years had refrained from directing his criticism of Russian politics at Putin. Putin, for his part, has been extremely critical of Gorbachev's legacy, blaming him for the Soviet Union's demise.

Putin has shown no sign now of moving to appease the protesters. On Wednesday he registered his candidacy for the March presidential elections, in which he seeks to win a third term.

Putin, 59, was president from 2000 to 2008, then switched to premier due to term limits. But even in that No. 2 slot, the steely Putin dominated Russian political life, overshadowing his mild-mannered and hesitantly reformist successor Dmitry Medvedev. His selection in March is virtually guaranteed, and it's possible he could lead Russia until 2024.

Harsh action by police against protesters this week also indicates authorities are unwilling to give any ground, and will continue to deny opposition groups permission for most rally permits and break up any unsanctioned gatherings.

Thousands of security forces were out in the Russian capital and helicopters roamed the sky Wednesday. At least 51,500 police officers and 2,000 paramilitary troops have been deployed in Moscow since the election, authorities say.

Squads of police lined the sidewalks around Moscow's Triumphal Square as opposition supporters tried to gather. Police pushed demonstrators back, seized some and dragged them into police vehicles. Demonstrators made several attempts to return to the square but were repeatedly repulsed. Moscow police spokesman Anatoly Lastovetsky said at least 20 people were detained.

In St. Petersburg, demonstrators gathered outside the Gostiniy Dvor shopping complex on the city's main avenue, many chanting ``Shame, Shame!'' Russian news reports said at least 70 people were detained.

A photographer taking pictures for The Associated Press was briefly detained after photographing police preparations around the Moscow square. He was released only after erasing the photos from his camera.

In the face of a huge police presence and the bone-wracking Russian weather, the protests are not likely to last long, said analyst Alexei Malashenko of the Carnegie Moscow Center.

``There's going to be no Tahrir Square; it's cold,'' he told The Associated Press, referring to the epicenter of the protests that brought down Egyptian President Hosni Mubarak. ``There will be a crackdown on opposition while the public will be given lots of promises with steps like changes in the government.''

But analyst Yuri Korgoniuk wrote in the respected online gazaeta.ru news site that ``authorities face a dilemma _ either to start a crackdown or ease the rules of political life.''

Korgoniuk said a full-scale crackdown could strain the state's resources but even easing political control would inevitably lead to the dismantling of Putin's ``power vertical,'' the powerful central control he established as president.

Authorities are also making efforts to rally government supporters.

About 1,000 people gathered at a pro-Putin concert Wednesday afternoon in central Moscow. The crowd of mainly young people waved Russian flags and danced as organizers spoke on a stage adorned with a banner reading ``The Future is Ours.''

Someone dressed as a giant white bear _ United Russia's mascot _ danced among the crowd. He stopped occasionally to hug supporters _ but kept right on dancing when someone ran out of the crowd to kick him in the rear.

College student Ivan Samburov, 17, said he had no interest in the protest rallies.

``I prefer just to stand like this and dance, to have fun and improve my mood, to feel that everything is good,'' he said.

The Russian Union of Journalists condemned police violence against protesters and called for an investigation into the dozens of attacks and arrests of journalists, describing them as ``an attempt to gag and intimidate society.''

On Wednesday, two video journalists working for the state-owned RIA Novosti news agency were briefly detained outside the Central Election Commission building.

The Kremlin scrambled Wednesday to explain how a profane criticism of popular Russian opposition blogger Alexei Navalny appeared on Medvedev's Twitter account. The message, a retweet of a message attributed to Duma deputy Konstantin Rykov, used a term of sexual disparagement against a blogger using the epithet ``party of thieves and crooks.'' It was a clear reference to Navalny, who popularized the term and was sentenced to 15 days in prison after being arrested Monday at a protest.

The Kremlin said in a statement the retweet was committed by a Medvedev technical staff member and ``the guilty will be punished.''