Russia's ruling United Russia party, whose majority was reduced in this month's parliamentary election, on Saturday backed Kremlin chief of staff Sergei Naryshkin to become speaker of the State Duma (lower house), the party said.

Naryshkin, 57, whose nomination had been widely expected, will take over from Boris Gryzlov, who quit in a move apparently aimed at cooling public anger over an election opponents say was rigged in United Russia's favour.

The speaker's post is seen as a nominal one in the top-down vertical of power designed by Prime Minister Vladimir Putin, who is expected to return to the Kremlin after a presidential election next March 4.

Naryshkin, like Putin a native of St Petersburg, worked in the 1980s in the economic attache's department of the Soviet embassy in Belgium. He speaks fluent English and French.

Since 2008 he has headed the Kremlin administration, where he was viewed as a trusted Putin aide capable of minding the presidency of Dmitry Medvedev, a Putin protege who is stepping down to switch roles and become prime minister.

Naryshkin's deputy, Vladislav Surkov, a key ideologist of Russia's political system, has taken over as the Kremlin's acting chief of staff.

United Russia won the December 4 parliamentary election, but its share of the official vote slipped to barely 50 percent from 64 percent in 2007, reducing its majority to just 13 seats.

The election results have been challenged by opposition parties that won entry to parliament and the extra-parliamentary opposition, who allege that United Russia's share of the vote was inflated by ballot stuffing and other electoral fraud.

Tens of thousands of people took to the streets of Russian cities last week to demand a rerun of the elections - the largest political protests during Putin's 12-year-old rule.

Parliament is due to reconvene on December 21 and hold a vote to elect the speaker. Medvedev will address lawmakers and other dignitaries in an annual address the following day.

A further round of election protests is planned for next Saturday, December 24, with the authorities giving permission for 50,000 to gather in Moscow.

(Reporting by Vladimir Soldatkin, Editing by Douglas Busvine and Mark Heinrich)