Mikhail Kasyanov on 16 January
Kasyanov says the authorities are afraid of the political contest
Russia's election officials have barred former Prime Minister Mikhail Kasyanov from running as an opposition candidate in the March presidential election.
The Central Election Commission (CEC) said there were many invalid signatures in Mr Kasyanov's list of supporters.
Mr Kasyanov was prime minister under President Vladimir Putin, but has become one of the Russian leader's staunchest critics.
Mr Putin's chosen successor, Dmitry Medvedev, is expected to win in March.
Russian law stipulates that no more than 5% of signatures in support of a candidate can be false or forged.
In Mr Kasyanov's case, 13.36% were rejected, the CEC announced. The decision was unanimous.
"This means the elections will be a farce," Mr Kasyanov's spokeswoman, Yelena Dikun, told Reuters news agency.
"It shows the authorities are afraid of any competition, it means the authorities fear any alternative points of view," she said.
"They are afraid, just so afraid."
Criticism fears
The CEC has registered three other candidates apart from Mr Medvedev - Gennadiy Zyuganov, Vladimir Zhirinovsky and Andrey Bogdanov.
Dmitry Medvedev, who is chairman of the gas giant Gazprom, has been leading recent opinion polls.
Mr Kasyanov, who would have been the only liberal running for the presidency, has been polling about 1%.
He says all this is part of a deliberate campaign by the Kremlin to crush his political ambitions.
And he has complained that his staff, including those who had to collect all the signatures from around the country, have been subject to a massive campaign of intimidation.
Although Mr Kasyanov stood absolutely no chance of winning the election, the Kremlin may have feared he could use the election campaign to criticise President Putin's record in office, says the BBC's Richard Galpin in Moscow.
And criticism is something the Kremlin seems increasingly unwilling to tolerate, our correspondent says.