Liberal leader Grigory Yavlinsky is likely to be excluded from the Russian presidential race, election officials said Tuesday, in a move that will undermine the legitimacy of Vladimir Putin's historic comeback.

The Central Election Commission said it had examined 500,000 of the two million signatures the veteran leader of the Yabloko party submitted to take part in March 4 presidential elections and found that nearly a quarter of them had problems.

"The number of signatures that have been deemed invalid and questionable gives the Central Election Commission grounds to deny registration" to Yabloko, Nikolai Konkin, a member of the election organisers, told reporters in televised remarks.

A commission spokesman told AFP officials would conclude examining a second and final sample of signatures later Tuesday before delivering their final decision.

The limit for rejected signatures is five percent. The commission has in the past rejected signatures in support of anti-Kremlin candidates on various grounds.

Analysts and opposition said Yavlinsky's likely exclusion from the race would further undermine the credibility of a vote already weakened by emerging claims of campaign violations and the fraud-tainted December parliamentary polls that observers said were slanted in favour of Putin's ruling party.

"This significantly weakens a list of candidates that has already been made artificially narrow and turns the election into a joke," said political analyst Gleb Pavlovsky, who used to work with the Kremlin.

Yabloko said the decision was politically motivated, adding that it expected further pressure from officials.

Organisers of the next in a series of anti-Putin rallies adopted a resolution Tuesday demanding that Yavlinsky be allowed to run. Yavlinsky himself plans to speak at the rally on February 4, the party said.

Prime Minister Putin is wrestling with the worst legitimacy crisis of his 12-year rule after he announced his plan to seek a third Kremlin term in a job swap with incumbent President Dmitry Medvedev in September.

His approval ratings took such a beating that he may not poll 50 percent, forcing him to take part in a run-off for the first time, a humiliating prospect for a leader who once enjoyed sky-high ratings.

Three more candidates fielded by political parties -- Gennady Zyuganov of the Communist Party, Sergei Mironov of A Just Russia party and Vladimir Zhirinovsky of the Liberal Democratic Party -- have been registered.

Independent candidate billionaire Mikhail Prokhorov, who critics say will run as a Kremlin-friendly candidate to soak up opposition to Putin, is most likely to be registered.

Prokhorov and Zyuganov came out in support of Yavlinsky, with the metals tycoon saying that the system that requires two million signatures for registration should be cancelled.

"The current procedure is dishonest, unfair, prohibitive and I'll even say humiliating," Prokhorov said Monday.

Political analysts and opposition said the Kremlin would seek to disqualify Yavlinsky from the race to ensure Putin's decisive victory in the first round, but questioned whether Yavlinsky's backers would switch to Putin.

"It appears that people in charge of the prime minister's campaign have lost their mind," Pavlovsky told AFP. "People voting for Yavlinsky would never vote for Putin anyway."

Yavlinsky, 59, a softly-spoken veteran liberal politician made a surprise comeback to politics after his Yabloko party polled well among middle-class voters in Moscow and Saint Petersburg during parliamentary elections.

The party still failed to make it into parliament in a vote that the opposition and observers said was skewed in favour of the ruling party.

Just one percent said earlier this month they would support Yavlinsky as president, compared to 52 percent who would vote for Putin, according to the state-controlled pollster VTsIOM.