The Russian government has sent police and Interior Ministry troops into Moscow after thousands of protesters took the streets Monday night accusing Prime Minister Vladimir Putin's party of electoral fraud.
Interior Ministry spokesman Vasily Panchenkov told the BBC they wanted "to ensure the security of the citizens".
Meanwhile, supporters of Putin's United Russia rallied Tuesday in Moscow to celebrate the party's victory in Sunday's vote and to counter the opposition protests.
"We will not let the Russian voting results be turned into a farce, and the free and democratic elections be discredited," members of the pro-Kremlin youth movement Nashi told RIA Novosti news agency. About 15,000 people gathered in central Moscow, carrying portraits of President Dmitry Medvedev and Putin.
On Monday night, in the biggest opposition rally in years, the police detained about 300 activists.
Estimates of the number of protesters ranged from 5,000 to 10,000, according to the AP.
Putin's United Russia party won about 50 per cent of Sunday's vote, which represent a sharp drop from the last election, when the party took 64 per cent.
The opposition politicians and monitors accused Putin of fraud at the ballot-box in order to inflate the vote's results, which represent a symbolic blow to United Russia.
The Organisation for Security and Co-Operation in Europe cited problems with the counting process. ""The contest was also slanted in favour of the ruling party, the election administration lacked independence, most media were partial and state authorities interfered unduly at different levels," said Petros Efthymiou.
U.S. Secretary of State Hillary Clinton also expressed "serious concerns" about the conduct of election.