The Russian prosecutor-general's office on Tuesday announced a forgery investigation has been opened against the campaign of an opposition candidate for the presidential election.

Prosecutor-general spokeswoman Tatiana Chernyshova said on state television that the case involves forged signatures on the petitions nominating Mikhail Kasyanov to run in the March 2 election.

If more than 5 percent of the signatures submitted by the campaign are forged, he cannot be registered to run, the ITAR-Tass news agency cited Central Elections Commission member Yelena Dubrovina as saying.

Kasyanov was a prime minister under President Vladimir Putin, but was dismissed in 2004 and later became a strong critic of the Kremlin.

A spokeswoman for Kasyanov's campaign, Yelena Dikun, said the office had not received any information from the prosecutor about the case, which she described as being "political pressure."

Kasyanov on Saturday issued a statement bitterly complaining of alleged official harassment, saying the interior and justice ministries "have conducted a coordinated campaign of pressure against me since I handed over the registration documents for the presidential elections.

"In numerous Russian regions, crude intimidation has been deployed against people officially registered as campaign workers ... people have been threatened, intimidated and forced to admit to committing illegal acts."

The final candidate list for the election is to be announced on Sunday.

Putin, who is constitutionally banned from seeking a third consecutive term, has endorsed First Deputy Prime Minister Dmitry Medvedev, giving him an overwhelming advantage.

Two other candidates have been registered, but neither present serious competition: Communist Party leader Gennady Zyuganov and Vladimir Zhirinovsky, head of the nationalist Liberal Democratic Party.

Russia's most visible liberal opposition figure, former world chess champion Garry Kasparov, intended to run but said his supporters were not allowed to rent halls for a nomination gathering — a requirement under Russian law for candidates that are not members of registered parties.

Chernyshova said suspected false signatures were found in the Yaroslavl and Mari-El regions.

The head of Kasyanov's campaign in Mari-El, Rustam Abdullin, was detained this month and found in possession of nominating petitions with suspected false signatures.