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 opposition forces to conduct a startling series of demonstrations, including a 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 would show that little vote fraud took place and that it had no effect on the outcome.
Peskov's comment signaled that Putin - who was Russia's president from 2000 to 2008 and moved 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 huge fortunes tycoons compiled as countless Russians struggled through the economic chaos of the 1990s after the Soviet Union collapsed. Prokhorov made his fortune in metals and banking and became majority stakeholder in the New Jersey Nets last year.
Kudrin, 51, lacks Prokhorov's flash, but as finance minister under 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.
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 Putin to run for another term. The decision by Medvedev and Putin to swap jobs was seen by critics as cynical and antidemocratic, so Kudrin's dismissal could give him a principled aura.
