MOSCOW - Russian prosecutors accused opposition candidate Mikhail Kasyanov on Tuesday of forging signatures for his presidential bid, a step that could pave the way for his disqualification from the election.

An aide to Kasyanov, whom opinion polls credit with only about one percent support, said prosecutors were applying "political pressure" in the build-up to an election likely to be dominated by President Vladimir Putin's choice, Dmitry Medvedev.

A former prime minister turned critic of Putin, Kasyanov had earlier predicted the authorities would try to sabotage his election bid.

European democracy watchdogs have urged Russia to ensure the presidential vote, set for March 2, is fair after Western observers accused the Kremlin of meddling in a parliamentary election last year.

First Deputy Prime Minister Medvedev is overwhelming favourite, buoyed by the backing of the popular Putin who is constitutionally barred from seeking a third term but seeks to extend his influence in the country.

Prosecutors said they had opened a criminal investigation over signatures Kasyanov submitted for his candidature.

Kasyanov served as Putin's prime minister until he was sacked in 2004. He submitted 2 million signatures supporting his bid - a legal requirement for independent candidates.

Tatyana Chernyshova, a spokeswoman for the Prosecutor- General's office, said prosecutors in two regions had uncovered evidence campaign workers forged thousands of signatures.

"A criminal investigation has been opened ... into the falsification of election documents," she told reporters. The investigation focused on the Volga River region of Mariy El, and the Yaroslav region in central Russia.

"POLITICAL PRESSURE"

Analysts say Kasyanov represents no threat to Medvedev in the election. But they say he is resented by some in the Kremlin because he broke a taboo by turning against his former colleagues and joining the opposition.

"This story with Mariy El has been going on for about a week. We believe it is political pressure," Kasyanov aide Yelena Dikun told Reuters. She said she had no information about the other investigation."

Nikolai Konkin, the secretary of the Central Election Commission, said a check of some of Kasyanov's signatures had shown over 15 percent were defective. More than 5 percent of rejected signatures is grounds for disqualification.

Itar-Tass news agency quoted Konkin as saying election officials would check another sample of the signatures before making a decision.

If Kasyanov is taken off the ballot, the field challenging Medvedev will be reduced to three: nationalist Vladimir Zhirinovsky, Communist leader Gennady Zyuganov and Andrei Bogdanov, head of the small Democratic Party.