Moscow: Russia's rebel billionaire Mikhail Prokhorov on Monday said he would challenge Vladimir Putin in the March presidential poll, tapping into the anger triggered by disputed legislative elections.

His declaration coincided with a call by a respected ally of Putin for Russia to create a new pro-business party that could focus on fighting corruption and regaining voters' trust.

The 46-year-old New Jersey Nets basketball team owner described his decision to run against Putin, an ex-KGB agent who has dominated Russian politics for 12 years, as "the most serious decision of my life".

His announcement came just days after more than 50,000 people massed near the Kremlin to protest the outcome of the December 4 elections in which Putin's United Russia managed to secure 238 seats in the 450-seat Duma, a sharp drop from the 315 it received in 2007.

Smaller demonstrations took place in cities across Russia to protest the vote that was marred by reports of violations and unprecedented state pressure against independent election monitors.

The White House on Monday hailed the protests as "a very positive sign to all those who support the democratic process" and "what appears to be a new era of tolerance for the freedom of assembly."

With a fortune estimated by Forbes magazine at USD 18 billion, Prokhorov has remained a mysterious figure who has navigated Kremlin politics and survived Russia's cut-throat business world to build a metals empire.

Once seen as a close Putin ally, Prokhorov made a short-lived effort to challenge Putin's United Russia party in this month's parliamentary elections, but resigned following an internal power struggle he blamed on the Kremlin.

"If you remember, the Kremlin removed me and my supporters from Pravoye Delo (Right Cause party), and we could not complete what we had set out to do. It is not in my nature to stop halfway," he said.

The Right Cause party finished with less than one percent of the vote in the December 4 polls.

Prokhorov, who has been largely absent from the media since September, said he had "been working quietly and calmly building the infrastructure that I need to collect the 2.5 million signatures" to register in the race.

"We have a parliament that is completely leftist," said Prokhorov, promising to build his programme "on a complete lack of populism" and reforms that would strengthen Russia's middle class.