Russian officials have barred liberal opposition leader Grigory Yavlinsky from running for president against Prime Minister Vladimir Putin because of alleged problems with his registration forms.

The central elections commission said Friday it could not accept about 25 percent of the signatures gathered to back Yavlinsky's candidacy because they are invalid.

The move increases Prime Minister Putin's chances of winning the presidential election in March.

On Monday, Yavlinsky challenged a threat by Russian election officials to bar him from the poll. He said a ban on his candidacy would insult tens of thousands of protesters who have recently taken to the streets of major Russian cities to demand political reforms and fair elections.

The 59-year-old economist needed to gather 2 million signatures to qualify to run against Mr. Putin, the overwhelming presidential favorite. The signatures were required because Yavlinsky's Yabloko party failed to win any parliamentary seats in December elections.

Also Monday, Russian officials said they are likely to register billionaire tycoon Mikhail Prokhorov, who announced his candidacy late last year.

Authorities said Prokhorov, a precious metals magnate, only has problems with about 4 percent of his signatures. The tycoon, who owns 80 percent of the (U.S.) New Jersey Nets professional basketball team, is not seen as a legitimate threat to Mr. Putin's presidential aspirations. He is seen in some quarters as running at the Kremlin's behest to make Mr. Putin's inevitable victory on March 4 look competitive.

Prokhorov rejects such criticism, saying he favors participation in the polls by all of Russia's political parties.

If Mr. Putin regains the presidency, the 59-year-old leader could serve two more six-year terms and remain in power until 2024. He was first elected president in 2000 and held that post until 2008, when he assumed the post of prime minister due to term limits.