President Dmitry Medvedev told the heads of parliament's four factions in a special meeting that the first session will be held on December 21 even while checks into various vote-fixing reports continue.
"Today I am signing a decree for the first session of the Duma to be held on December 21," Medvedev said. "Far from everyone is happy with the election results ... but that is always the case."
The decision ends any opposition hopes of forcing the authorities to call a re-run of a December 4 vote that handed strongman Vladimir Putin's ruling United Russia part a slim majority, despite sharply reduced support.
The outcome was accompanied by a flood of footage shot by voters at polling stations and posted on the Internet appearing to show ballot boxes filled with votes before polling began hours and officials filling in fake results by hand.
The vote had been seen as a test of Putin's decision return to the Kremlin in March presidential elections and the worse-than-expected result and subsequent protests appear to have caught the state off guard.
Medvedev said he still intended to deliver his annual address to the nation on December 22 -- two days before a mass opposition rally in Moscow is expected to draw some 50,000 people for the second time in two weeks.
Putin is also due to address the worst political flare-up of his 12-year rule in an annual televised question and answer sessions with ordinary Russians on Thursday. But his spokesman said most Russians who had already sent in their questions and clearly had other pressing concerns on their mind.
"Only five per cent are asking about politics and four per cent about human rights," Putin's spokesman Dmitry Peskov told reporters. Medvedev for his part promised to push ahead with political reforms before leaving office in March after just one term.
"We must take new decisions, take more decisive steps to remove barriers on political activity. ... And what is most important for our country, (we must) bridge the gap between various social groups and government institutions."
United Russia is expected cede control of 14 of parliament's 29 committees -- a step aimed at giving the three nominally opposition parties in parliament a stronger voice. The small gesture should not complicate the Kremlin's ability to smoothly pass its own legislation through the chamber.
But it marks the end of United Russia's monopoly on parliamentary power and highlights the rising discontent against ex-KGB agent Putin himself. Former minister turned Kremlin critic Boris Nemtsov said Medvedev's decision to convene parliament will not put a halt to the street protests. "We will still be pressing the same demands," Nemtsov told AFP. "It is not too late. The people do not recognise this Duma's legitimacy."
Analysts believe Putin should still win the presidential polls but predict a far rockier ride to his third Kremlin term than appeared the case just weeks ago when the opposition lacked the confidence it gained with the rallies.
An unexpected challenger appeared Monday in the shape of tycoon Mikhail Prokhorov -- a political novice who has shown liberal sympathies. Some newspapers quickly expressed suspicions that his candidacy was no more than a Kremlin plot to soak up dissent.
The Vedomosti daily quoted a Kremlin source as saying that Prokhorov's candidacy was a "tactical decision" aimed at reducing tensions in society after the protests of the last week. The ultra-nationalist Liberal Democratic Party Tuesday meanwhile nominated its leader Vladimir Zhirinovsky to stand in the elections for a fifth time.
Sergei Mironov of A Just Russia -- a party with a pro-Kremlin voting record despite its official opposition status -- also formally put in his candidacy application with the central election commission. Mironov predicted a close race in which Putin will be force into a runoff due to quickly-dropping support.
"I am convinced that the election will require two rounds," Interfax quoted Mironov as saying.
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