A party congress in Moscow overwhelmingly approved by a show of hands Medvedev's candidacy to take over from President Vladimir Putin as party leader, with no apparent opposition and no other candidate contesting the post.

Medvedev's move to become a card-carrying party leader is a key plank in Russia's new power structure that saw him cede the Kremlin to Putin earlier this month after serving four years as president.

In a high-orchestrated congress, Putin formally announced Medvedev's candidacy at central Manezh hall next to the Kremlin to the crowd of 3 000 guests and delegates.

"I consider United Russia my key ally and partner," said Putin, dispelling speculation that the party has been sidelined by the Kremlin and may even be disbanded after it lost support in December parliamentary elections.

Putin has been the leader of the party since 2007, although curiously never joined. He decided to fully step aside in April after announcing that leading the party is inappropriate for the president.

However Medvedev this week formally joined the party, vowing to reform it from the inside.

He said during the congress the party should undergo "revolutionary" changes in order be "more understandable to people" rather than seem "imposed from above" and vowed to cleanse it of unworthy members.

"United Russia has been in power for a considerable amount of time," Medvedev said. "There is a factor of tiredness beginning to play out against it."

Dominating the Russian parliament since 2003, the party supported Vladimir Putin in his second election into the presidency in 2004 and has essentially been a rubber-stamping tool of the Kremlin.

Last year the opposition waged a campaign ahead of the parliamentary polls asking voters to cast their ballots for any party but United Russia, dubbing it the "party of crooks and thieves" and inundating Internet blogs with videos and posters discrediting the faction.

As a result United Russia received only 49.3% of the popular vote, hanging on to more than half of the parliament seats only by a quirk of Russia's electoral law.