Thousands of interior ministry troops and police were deployed on the streets of Moscow Tuesday, following one of the biggest liberal opposition rallies Russia has seen in years.

Eyewitnesses posted reports on social networking sites of buses filled with military personnel arriving in the center of the capital, Ria Novosti news agency reported.

Up to 4,000 troops and police were deployed in Moscow, a city police spokesman told Agence France Presse.

Security forces will remain on heightened alert across the country until all votes have been counted, Interior Ministry spokesman, Oleg Yelnikov, told AFP. The authorities are trying to "ensure the security of the citizens," said another spokesman, Vasily Panchenkov.

At least 300 protesters were arrested in Moscow Monday night as they demonstrated against alleged fraud in Russia's parliamentary elections, according to Ria Novosti. Another 120 were detained at a similar rally in St. Petersburg.

Some of those arrested, including anti-corruption blogger Alexei Navalny and opposition activist Ilya Yashin, were due to appear in court Tuesday. They face 15 days in jail for refusing to obey police orders.

Several thousand people took part in Monday night's rally in central Moscow. The figure police gave was 2,000, but the Moscow Times said its reporters had seen between 5,000 and 7,000 protesters.

According to the Guardian's correspondent, the crowd had been attending an opposition event sanctioned by the city authorities, but trouble started when they deviated from the program:

When they attempted to march to Lubyanka, the headquarters of the feared Federal Security Service, they were met by baton-wielding riot police who beat some protesters over the head and dragged others into waiting lorries.

This video of the rally shows a large crowd of people marching and chanting slogans. Protesters shouted "We need new elections," "Russia without Putin" and "Revolution," according to the Moscow Times.

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Supporters of the ruling United Russia party staged rival demonstrations in Moscow Tuesday, defending the election results against accusations of fraud.

According to police, some 15,000 members of the pro-Kremlin youth movement Nashi attended an event Tuesday afternoon, while 10,000 were expected at a so-called "Clean Victory" rally Tuesday night.

Opposition activists are calling for a second rally of their own Tuesday night via social networking sites, the BBC reported.

International observers have questioned the official election results, which gave United Russia a narrow majority. The Organisation for Security and Co-operation in Europe (OSCE) said its observers had reported "serious indication of ballot box stuffing," while US Secretary of State Hillary Clinton said Washington had "serious concerns" about the vote and called for a full investigation.

President Dmitry Medvedev had dismissed the allegations, saying that the elections were "absolutely fair and just."

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