MOSCOW — There were no balloons or streamers at the hastily arranged news conference on Monday where Mikhail D. Prokhorov announced that he would challenge Vladimir V. Putin in this spring’s presidential elections: just Mr. Prokhorov on the podium, looking grave.

Mr. Prokhorov, the billionaire who is majority owner of the New Jersey Nets basketball franchise, became famous in Russia as a playboy, with the tabloid nickname “the Holiday Man.” He has long walked a fine line in his relationship with the Kremlin, maintaining a degree of independence while rebels among his fellow oligarchs ended up in exile or in prison.

Mr. Prokhorov’s announcement, which prompted gasps from reporters in the room, was the latest wild card in a week that seemed to return real politics to Russia. Tens of thousands of middle-class urbanites gathered Saturday to vent their anger over tainted and noncompetitive elections. Aleksei L. Kudrin, a longtime Putin adviser and former finance minister, declared Monday that he plans to help found a party to represent reform-minded voters.

Mr. Prokhorov, 46, has the resources to pose a real challenge to the Kremlin if he chooses. Over the years, he has sometimes made accommodations — like firing the editor of a newspaper he co-owned that had obliquely criticized Mr. Putin. Other times he has pushed back, as in September, when he clashed with Kremlin political strategists and was unceremoniously kicked out of his pro-business party, Right Cause. He spent the next two and a half months in a kind of political exile, until Monday, when he said he would run for president.