Russian investigators on Saturday said they had launched a probe into multiple violations during last month's parliamentary polls, ahead of a new report by Western observers.
Since the December elections, Russians have held unprecedented mass rallies against vote fraud in a major challenge to Prime Minister Vladimir Putin's rule as he seeks a return to the Kremlin in March elections.
Russia's Investigative Committee said in a statement Saturday it had opened 26 criminal cases into possible violations during the December 4 polls, including ballot stuffing and bribing voters.
There had been 350 reports by the public of possible violations, it said.
"Based on preliminary findings, investigators have currently launched 26 criminal cases" into possible violations of voters' rights, falsified election documents and rigged poll results, the statement said.
"The main violations have to do with stuffing ballots, bribing voters, and forcing them to vote, among others," it said.
Despite the mass protests of recent months, demands to investigate reports of violations have so far yielded few results.
Central election commission chief Vladimir Churov has alleged that amateur video clips featuring the stuffing of ballot boxes were filmed at private apartments decorated to resemble polling stations.
Western observers with the monitoring mission at the December parliamentary polls from the Council of Europe Parliamentary Assembly were later Saturday to present findings from their visit to Russia.
Previously the Organisation for Security and Cooperation in Europe (OSCE) has said that the polls were marred by frequent violations, many of which had occurred after polling stations closed.
The Russian Investigative Committee said it is looking into fraud claims in six regions, including Moscow, and that "practically all parties" had been involved in orchestrating it, without listing the names of the parties.
One party had paid a woman 9,000 roubles and she had then proceeded to bribe people, paying them 300 roubles each, the committee's statement said, promising an "objective and comprehensive investigation".