Russian prosecutors have opened an investigation which is expected to disqualify presidential opposition candidate Mikhail Kasyanov for allegedly forging the signatures he needed to run, Interfax news agency reported Tuesday.

Former prime minister Kasyanov said any move to block his joining the ballot was a case of "political pressure."

"I don't imagine a scenario in which I would not be registered. If this decision is taken - it will be a political one," Kasyanov told reporters Tuesday.

Kasyanov, who turned critical of President Vladimir Putin after being dismissed in 2004, had been viewed as the only opposition candidate in the race, though he trails far behind the favourite Dmitry Medvedev.

Medvedev, President Vladimir Putin's anointed successor, was in the throws of his first day of official campaigning Tuesday after being automatically registered as the pro-Kremlin United Russia party candidate.

During a nine-minute televised speech shown on Russia's NTV news segment, Medvedev launched his campaign, calling for a "war on corruption."

Medvedev, who is in charge of Putin's so-called National Projects to reinvest Russia's oil funds into social programmes, said he would raise fighting corruption to the priority of a national project.

Kasyanov, as an independent candidate, filed the required two million signatures in support of his self-nominated to the March 2 presidential vote.

Central Election Commission (CEC) secretary Nikolai Konkin said that, while the lists were still being verified, an initial check of 400,000 signatures found over 15 per cent were invalid - much more than the 5 per cent barrier for disqualifying a candidate.

CEC member Gennady Raikov said Tuesday that it was "improbable, but a chance remained" for Kasyanov to be registered.

The additional announcement by the General Prosecutor's office Tuesday that it was leading a criminal investigation into forgeries in support of Kasyanov's bid seemed to his chances of appearing on the ballot.

Prosecutors charged that 12,000 signatures collected in the Volga region of Mari El had been forged, spokeswoman Tatyana Chernyshova was quoted by Itar-Tass as saying.

The head of the State Duma Legislative Committee, Pavel Krasheninnikov said the investigation of falsifications would not lead to Kasyanov's personal prosecution, but may lead to rejection of his registration.

Kasyanov said Tuesday he was aware he risked arrest, but that he would "fight to the end."

Meanwhile, the election commission on Monday officially registered First Deputy Prime Minister Medvedev, who is exempt from submitting signatures because he was nominated by two parties in parliament for the March 2 presidential vote.

As of Tuesday, three candidates were registered for the race including two other candidates fielded by their parties: Communist leader Gennady Zyuganov and Vladimir Zhirinovsky, head of the ultranationalist Liberal Democratic Party.

The election commission is also reviewing the signature list of the head of the Democratic Party of Russia Andrei Bogdanov, who is the only other self-nominated candidate looking to join the ballot.

Critics have charged that Bogdanov's candidacy is part of a Kremlin project to divide the opposition and point out that the party received about 90,000 votes in last month's State Duma election, much less than the 2 million needed to enter the presidential race.

Observers believe that the paperwork required for registration provides an opportunity to reject Kasyanov's registration on technical grounds if the Kremlin decides against his running.

If successful, Kasyanov will be the only opposition figure from the activist anti-Kremlin political movement that organized a series of so-called Dissenters' Marches in the run-up to December parliamentary elections.

Former chess hero Garry Kasparov pulled out of the race in December saying he was blocked by authorities from holding a congress for his nomination.

With Putin's backing Medvedev's victory is widely viewed as a forgone conclusion.

According to Russia's state pollster on Monday, Medvedev was supported by 60 per cent of voters, Zhirinovsky by 8 per cent and Zyuganov by 6 per cent.

Kasyanov had 1 per cent of voter's support, according to the nationwide poll of 1,600 voters. The poll's margin of error was 3.4 per cent.