Russia's ruling party has appointed Prime Minister Dmitry Medvedev as its new leader at a congress in Moscow.
The former president succeeds his ally, President Vladmir Putin, who nominated him for the job in April.
Correspondents say that Mr Medvedev is taking over a party which finds itself in deep crisis.
Despite suspicions of widespread vote rigging, United Russia lost a quarter of its seats in the parliament after the elections in December.
Mr Medvedev, who was elected unopposed by a show of hands, called for more democracy within the party and for all party positions to be chosen by secret ballot.
Officials admit that a comprehensive revamp is badly needed before the upcoming season of regional elections.
The prime minister said United Russia had to change with the times.
"The party is already old and the political system has become more and more varied. So it won't be easy. That means we have to ready to answer all challenges. To keep power, respond to criticism, argue, .... It seems to me that together we can achieve that."
The BBC's Leonid Ragozin in Moscow says the party is widely perceived as a network of political opportunists whose businesses depend on proximity to the government.
He says Mr Medvedev will have to fight an uphill battle if he wants to reform a party that many Russians refer to as the "party of crooks and thieves".
Mr Putin, 59, was sworn in for his third term as president earlier this month, and formed his government this week.
Mr Medvedev entered the Kremlin in 2008 because of a bar on Mr Putin serving three consecutive presidential terms.