Russian tycoon Mikhail Prokhorov says he will run against Prime Minister Vladimir Putin in a presidential election next March, in the latest challenge to the Russian leader.

Prokhorov, who also owns the U.S. basketball team the New Jersey Nets, said Monday his decision to compete in the election is the most important of his life.

Prime Minister Putin has seen his authority undermined by nationwide mass protests against a December 4 parliamentary election that opposition activists say was rigged in favor of his ruling United Russia party. Tens of thousands of people rallied in Moscow on Saturday to demand an election re-run. It was one of the largest anti-government protests in Russia since the 1990s.

Mr. Putin is running to reclaim the presidential post he held from 2000 to 2008 and appoint his chosen successor, Dmitry Medvedev, as prime minister. The proposed job swap also has angered many Russians.

President Medvedev said Sunday authorities will investigate alleged manipulation of the parliamentary election. But Mr. Putin's spokesman, Dmitry Peskov, said Monday the alleged fraud affected less than 1 percent of the total votes cast and predicted the cases will not affect the election's legitimacy.

Mr. Putin's United Russia saw its share of the vote drop to just under 50 percent in the election, significantly reducing the party's majority in the lower house of parliament. But critics say that result still was inflated by fraud.

In another development, a former Putin aide has called for the creation of a liberal party to offer voters an alternative to United Russia. In an interview with Russian business daily Vedomosti published Monday, former finance minister Alexei Kudrin said the demand for such a party is “so high that it certainly will be established.” He also expressed a willingness to participate in its creation.