Russian election officials say liberal opposition leader Grigory Yavlinsky could be barred from running for president against Vladimir Putin in upcoming elections, because of alleged problems with his registration forms.

Yavlinsky on Monday challenged the threatened disqualification, saying 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 needs to gather 2 million signatures to qualify to run against Mr. Putin, the overwhelming presidential favorite who currently holds the post of prime minister. The signatures are required because Yavlinsky's Yabloko party failed to win any parliamentary seats in December elections.

Election officials said Monday that nearly 25 percent of Yavlinsky's signatures were found to be invalid, but said they will check more names before ruling on the application.

Officials also said they are likely to register billionaire tycoon Mikhail Prokhorov, who announced his candidacy late last year.

Authorities say 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.