With 98% of Sunday's vote counted, preliminary results give the pro-Kremlin United Russia party 64.1%, the Communists 11.6%, the ultranationalist Liberal Democratic Party 8.2%, and the loyalist A Just Russia 7.8%.
A joint statement by the Organization for Security and Co-operation in Europe (OSCE) and the Council of Europe said Sunday's poll "was not fair and failed to meet many OSCE and Council of Europe commitments and standards for democratic elections."
Goran Lennmarker, president of the OSCE's Parliamentary Assembly, said Russia's new election law, which raised the threshold for entry to the State Duma from 5 to 7%, and also canceled "the against all" option and minimum voter turnout requirements, had created unequal conditions for smaller political parties, and therefore the elections could not be described as fair.
He also said opposition parties had been pressurized. Shortly before the polls, two anti-Putin meetings in Moscow and St. Petersburg were dispelled by riot police and several opposition figures arrested. One of these, Garry Kasparov, was subsequently jailed for five days.
Referring to President Putin's decision in October to head the United Russia candidate list, Lennmarker said the Kremlin's virtual merger with the party was unacceptable and clearly violated international norms.
Another criticism leveled at United Russia was that the party had enjoyed almost unlimited access to funds, its election campaign dwarfing the attempts of other parties to gain a platform.
Central Moscow was dominated by United Russia adverts in the days before the election, with little or no sign of the existence of other parties.
The run-up to the Duma elections was marred by a dispute with the OSCE, after its main election monitoring arm refused to dispatch observers for the polls, citing restrictions and visa delays. Moscow in turn criticized "chaos" in the organization and reinvigorated calls for its reform.
Luc van den Brande, the chief Council of Europe delegate, added that the Duma polls were more of a referendum for President Vladimir Putin than a parliamentary election.
Igor Borisov of Russia's Central Election Commission said the OSCE and Council of Europe allegations were ungrounded and that the commission had not received any official reports from these organizations.
"I believe these conclusions were based on statements from those political leaders who were not satisfied with their parties' election results," Borisov said.
Kimmo Kiljunen, vice president of the Parliamentary Assembly of the Organization for Security and Cooperation in Europe (OSCE), said earlier on Monday that the voting had been fair and democratic, but added that that was merely his personal opinion.
Observers from the Commonwealth of Independent States, a Russia-led alliance of former Soviet republics, stated on Monday in a report that the Duma elections had been democratic, free and transparent.
