MOSCOW - After a week of surprising challenges to his authority, Vladimir Putin faces a new one from one of Russia's richest and most glamorous figures: the billionaire owner of the New Jersey Nets says he will run against him in March's presidential election.

The announcement Monday by Mikhail Prokhorov underlines the extent of the discontent with Putin, who has dominated Russian politics for a dozen years - first as president, then as prime minister.

It comes on the heels of Saturday's unprecedented nationwide protests against Putin and his party, United Russia. Tens of thousands of people gathered in the streets to denounce alleged election fraud favoring United Russia in Dec 4 parliamentary elections.

The fraud and the party's comparatively poor showing in the elections - losing about 20 percent of its seats, although it retained a narrow majority - galvanized long-marginalized opposition forces to conduct a startling series of demonstrations, including an enormous rally of at least 30,000 in Moscow alone.

In yet another challenge to Putin, his former finance minister, Alexei Kudrin, said Monday he was ready to work to form a new party.

At a news conference announcing his candidacy, Prokhorov refrained from criticizing Putin or President Dmitry Medvedev, but he said "society is waking up."

"Those authorities who will fail to establish a dialogue with society will have to go," he declared.

Medvedev has promised on his Facebook page that the alleged vote fraud will be investigated. But Putin's spokesman, Dmitry Peskov, predicted Monday the probe will show that little vote fraud occurred and that it had no effect on the outcome.

It is unclear how effective a challenger Prokhorov might prove to be. His wealth, estimated by Forbes magazine at $18 billion, and his playboy reputation may turn off voters who resent the gargantuan fortunes compiled by tycoons even as countless Russians struggled through the economic chaos of the 1990s.

The 46-year-old bachelor is known for lavish parties and occasional scandal. He and some guests were arrested at a Christmas party in the French Alpine resort of Courchevel in 2007 for allegedly arranging for prostitutes; but he was soon released without charges.

Prokhorov made his fortune in metals and banking and became majority stakeholder in the New Jersey Nets last year. Since then, he has traveled widely to build a global fan base for the team, in the process showing off his towering 6-foot-8 (203-centimeter) frame and excellent command of English.

Asked if he thought Prokhorov could run a country, Nets coach Avery Johnson said he had many qualities.

"He is pretty smart. he has great leadership skills. When you are behind the scenes and you are talking to him, you know he is a special person. It wouldn't surprise me," Johnson said.

"If we could vote, he would have a lot of votes here in this building."