MOSCOW (AP) ― 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.

Peskov’s comment signaled that Putin ― who served as Russia’s president in 2000-2008 and stepped over to the premiership because of term limits ― is holding firm, despite the protests that were the largest in post-Soviet Russia.

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 after the Soviet Union collapsed.

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 basketball team, in the process showing off his towering 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.”

Kudrin, 51, lacks Prokhorov’s flash, but as finance minister under both Putin and Medvedev, he earned wide respect for his economic acumen. Kudrin was widely credited with softening the blow of the 2008-09 global downturn in Russia with his conservative fiscal policies.

During Putin’s presidency, Kudrin set up a rainy day fund of revenue from Russia’s oil exports. The idea angered many in the government who sought higher spending, but it ultimately proved to be an invaluable cushion.

In an interview with the business newspaper Vedomosti published Monday, Kudrin said the country needed a new liberal party and “I am to assist” in creating it.

Kudrin was fired in September for saying that he would not serve if Medvedev agreed to step aside, become prime minister and allow the 59-year-old Putin to run for another term.

The decision by Medvedev and Putin to effectively swap positions was seen by critics as cynical and antidemocratic, so Kudrin’s dismissal could give him a principled aura.

Prokhorov said he hopes to win the support of Russia’s growing middle class, which formed the core of Saturday’s demonstrations. However, he said he agrees with only some of the anti-Putin and anti-government slogans shouted at rallies. He also did not say whether he plans to attend a follow-up protest in Moscow later this month.

He is one of several candidates who have said they will oppose Putin in the presidential election, including Communist chief Gennady Zyuganov, who has finished second in past presidential elections.

Prokhorov’s presidential bid follows his botched performance in the parliamentary race when he formed a liberal party under tacit support of the Kremlin, then abandoned the project under what he called Kremlin pressure.

He has personally blamed Vladislav Surkov, a presidential deputy chief of staff, for staging a mutiny within that party’s ranks. “I can solve that problem by becoming his boss,” Prokhorov said, referring to Surkov’s possible opposition to his candidacy.

Prokhorov now faces the immediate challenge of collecting the 2 million signatures required to qualify for the race. A number of opposition candidates and parties in the past could not even run for parliament because their applications were turned down for technical reasons.

Prokhorov also is not the first of Russia’s superrich to have ambitious political goals. Mikhail Khodorkovsky, once Russia’s richest man, has been in prison since 2003 on tax evasion and embezzlement charges that are widely seen as a punishment for having challenged Putin’s power.

The demonstrations in Moscow and other cities were “a very positive sign for all those who support the democratic process,” White House spokesman Jay Carney said Monday. “Both the Russian government and civil society leaders seem to be looking for a dialogue.”

The influential Russian Orthodox Church has also weighed in on the brewing controversy over the elections.

“Very serious questions have been raised, however uncomfortable for the authorities. We will hope that the authorities respond to them adequately and honestly,” church spokesman Vsevolod Chaplin told the Interfax news agency.