Billionaire Mikhail Prokhorov said he’ll run for president against Prime Minister Vladimir Putin in March elections after the biggest anti-government demonstrations in a decade emboldened Russia’s opposition.

“This is the most important decision of my life,” the New Jersey Nets basketball team owner told reporters in Moscow.

Prokhorov, Russia’s third-richest man with a fortune Forbes magazine put at $18 billion, said that he’ll seek to build support from the grassroots level and that he opposes “revolution” and “populism.” Prokhorov, 46, quit as leader of the Pravoye Delo, or Right Cause, party on Sept. 15, accusing President Dmitry Medvedev’s administration of blocking the group’s preparations for parliamentary elections in December.

That Dec. 4 vote, in which Putin and Medvedev’s United Russia party retained its majority, was neither free nor fair, observers from the U.S. and Europe said. Thousands of Russians took to the streets in the week after the contest to protest the results amid widespread reports of ballot-stuffing. Police estimated the crowd at Moscow’s Dec. 10 rally, the largest in the country, at 25,000, the same figure they ascribed to a pro- United Russia rally near Red Square today.

READ MORE: BLOOMBERG