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