Former Soviet leader Mikhail Gorbachev has called for the Russian parliamentary vote to be anulled.
In the face of mounting protests over accusations of election fraud, Gorbachev told the INterfax news agency that a new election should be held.
Hundreds of arrests were made in Moscow following public protests over the election, which saw Vladimir Putin's United Russia Party lose its grip on an overall majority.
"The results do not reflect the will of the people," Mr Gorbachev, president when the Soviet Union collapsed two decades ago, told the Interfax news agency.
"Therefore I think they (Russia's leaders) can only take one decision - annul the results of the election and hold new ones."
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.
Russia's foreign ministry has dismissed US criticism as "unacceptable", according to Reuters.