Dmitry Medvedev was elected as Russia's next president, early results showed on Sunday, after a vote that will preserve the power of his mentor President Vladimir Putin but which opponents said was unfair.

Medvedev, a 42-year-old former lawyer who has worked at Putin's side since the 1990s, will take over the trappings of the presidency from his patron in May but it was still unclear which of the two men would really be in charge.

Showing off the double act that will be at the helm of the vast, nuclear-armed country, Medvedev and Putin walked side by side out of the Kremlin gates and climbed onto the stage at a victory concert on Red Square.

"We can preserve the path that President Putin has proposed and I am convinced that we have every chance of doing that", a smiling Medvedev, dressed in jeans and a black leather jacket, said from the stage.

Opponents complained the contest was a stage-managed by Putin, a former KGB spy. "This is a secret service KGB operation to transfer power from one person to another," said ex-premier Mikhail Kasyanov, who was disqualified from the ballot.

But Russia voters are enjoying the biggest economic boom in a generation and most see Medvedev as the best hope of prolonging their new prosperity.

He has asked the 55-year-old Putin to be his prime minister and promised to continue his mentor's policies.

"I think that he (Putin) will, at least at the beginning, be the key figure," said Sergei Markov, a Kremlin-linked political analyst and member of parliament.

An exit poll by state-run pollster VTsIOM gave Medvedev 70.1 percent of the vote. Communist challenger Gennady Zyuganov was in second place with 16.8 percent.

In the official count, Medvedev was leading with 68.19 percent after ballots from half of the country's polling stations had been counted.

The outgoing president, barred by term limits from running in this election, will step down in May and move the short distance down the Moskva river to the prime minister's office in the government headquarters, known here as the White House.

The handover has been smooth for now but observers question how Putin and Medvedev will make their double act work in a country which is used to having a single, strong ruler.

ONE-SIDED

Medvedev's victory follows a one-sided campaign in which the former corporate lawyer refused to take part in televised debates and enjoyed lavish coverage on state television.

Democratic U.S. presidential candidate Barack Obama, said in a comment supplied to Reuters: "I am disappointed that this election was not fully free and fair because of the absence of a free media, and the harassment of opposition figures and political parties."

Putin though told the Red Square concert the vote was in "strict accordance with Russia's constitution." Election officials said they had no evidence of violations that would cast doubt over the result.

Medvedev's victory has been seen as a foregone conclusion since the popular Putin -- credited at home with restoring Russia's might on the world stage and stamping out the chaos of the 1990s -- endorsed him for the Kremlin top job last year.

Investors are watching closely to see who will be in Medvedev's government, with the focus on whether Finance Minister Alexei Kudrin, seen as the architect of financial stability, will keep his job.

Kremlin chief of staff Sergei Sobyanin said he doubted there would be any major shake-up in the cabinet before Medvedev is sworn in as president on May 7.

Russian gas giant Gazprom , of which Medvedev is chairman, was preparing to reduce supplies to pro-Western neighbour Ukraine over a debt dispute at 0700 GMT on Monday, in what some observers see as a show of strength.