Russia's election authorities have barred former prime minister Mikhail Kasyanov from running in a March presidential election against President Vladimir Putin's chosen candidate.
The Kremlin critic's supporters said the 2 March poll would now be a farce and the move showed the Kremlin feared any real challenge to Dmitry Medvedev, the man Mr Putin has backed to succeed him.
The Election Commission voted unanimously to refuse to register Mr Kasyanov because of technical errors, including forgery, which it said were found in some of the two million signatures needed to register him as a candidate.
Mr Kasyanov, 50, who served as Mr Putin's first prime minister, has accused his former boss of rolling back democracy, mismanaging Russia's booming economy and creating a political system that is dangerously dependent on one man.
But he had little chance of winning an election, with opinion polls showing he had less than 1% support.
Blocking Mr Kasyanov from the race may raise concerns about the fairness of the vote, which the opposition says is slanted in favour of Mr Medvedev - a 42-year-old first deputy prime minister who polls forecast will win a landslide in the election.
The election will be closely monitored by the US and the EU after international observers said last year's parliamentary vote was skewed by interference from the authorities.
Vladimir Putin, 55, who is unable to seek a third successive term under constitutional limits, has endorsed Mr Medvedev and is expected to remain influential after the election.
Four candidates are to take part in the presidential election: Mr Medvedev, Communist Party leader Gennady Zyuganov, firebrand nationalist Vladimir Zhirinovksy and Democratic Party leader Andrei Bogdanov.