Moscow ( CNN ) -- Russian billionaire Mikhail Prokhorov -- the owner of the New Jersey Nets basketball team and one of the world 's richest men -- said Monday that he will run for president of Russia next year . Calling his decision to run for president `` probably the most important decision of my life , '' he acknowledged the risks of challenging Russia 's rulers . `` There is saying in Russia : Never say never , anyone can end up behind bars . But I am not afraid , '' he said in a press conference in Moscow . Prokhorov , 45 , is worth $ 18 billion , Forbes estimated in March , making him Russia 's third richest man . Profile : Who is Mikhail Prokhorov ? His announcement set off a frenzy of speculation about whether he was running with the Kremlin 's tacit backing , to give the impression there was a genuine contest for president , or whether he really seeks to defeat Prime Minister Vladimir Putin , who also says he will run . He told reporters he had not discussed his decision with Putin or President Dmitry Medvedev , state-run news agency RIA Novosti reported . Prokhorov , who has dabbled in opposition politics before , wrote just last week that Putin was the only person who could run the country . `` You may like it or not but Putin is the only person who can somehow manage this ineffective state machine , '' he wrote on his LiveJournal blog . A Prokhorov representative declined a CNN interview request on the businessman 's behalf , saying Monday he wanted to focus on communicating with Russians via Russian media . Prokhorov 's Onexim Group has interests in metal , financial services , media , real estate , utilities and high-tech businesses , according to the Nets . He weathered the 2008 global financial meltdown particularly well , and told CNN the following year it was partly due to luck . But , he also said , `` I like business . It 's my profession . I spend , like , 15 hours a day in the office . It 's the great joy in my life . And I never think about money . '' He laughed when CNN 's Matthew Chance asked him what he thought of his reputation as a playboy . `` I do n't care . I try to act natural , '' he said , adding he had `` no time '' to think about having a family . Forbes magazine described him as a `` six-foot-eight bachelor and martial arts buff '' in March , when it listed him as the 32nd richest man in the world . Separately , Putin backers demonstrated in support of his government Monday , after enormous crowds protested against the government over the weekend . Tens of thousands turned out Saturday to protest against election results that returned Putin 's United Russia party to power . Police estimated crowds in Moscow at 25,000 , while organizers said it would be 40,000 , the state-run RIA Novosti news agency reported . Either figure would make the protests the largest in the Russian capital for decades . Claiming the results of parliamentary elections were rigged , protesters chanted `` Putin out , '' according to RIA Novosti . Protesters also braved freezing temperatures in other Russian cities to demonstrate against what they said was vote fraud . A day after the protests , Medvedev called for a probe into the allegations . `` I agree neither with the slogans nor the statements voiced at the protests , '' the statement on Medvedev 's Facebook page said . `` Nevertheless , I have ordered checks into all the reports from polling stations , regarding the compliance with the election laws . '' Critics piled ridicule on the Facebook post . `` It 's awful , '' Elena Panina wrote . `` Really . He was drunk when he wrote that or he did n't read what he wrote . ... What slogans does our president disagree with ? 'We are for fair elections ? ' '' Vladimir Kaganovich said the statement showed Medvedev is in the wrong job . `` The president of the country is n't an individual person and does n't have a right to show his disagreement with dozens of thousands of citizens , '' Kaganovich wrote . And he asked what it meant to launch a probe with no investigators and no time limit on the investigation . `` Please forgive me but it I would n't trust you even to manage a group in my department , '' Kaganovich wrote . And , addressing the president in the formal Russian manner , Sergey Pavlyuchenko simply wrote : `` Dear Dmitry Anatolievich ! Are you ever ashamed ? '' Putin 's United Russia party suffered big losses in the election but retained its parliamentary majority , according to official results . The protesters demanded an annulment of the December 4 election and a new vote . Around 7,000 people rallied in St. Petersburg , Russia 's second largest city , RIA Novosti said , citing police . Hundreds of protesters were arrested during demonstrations last week . Police said they cracked down on those demonstrations because the turnouts were not legal and the protesters were being disorderly . Election officials on Friday released the official election results : 238 seats for United Russia ; 92 seats for the Communists ; 64 seats for Fair Russia ; and 56 seats for the Liberal Democrats . Putin has announced plans to run for president -- the office he used to hold -- when Medvedev 's term expires early next year . CNN 's Arkady Irshenko and Phil Black contributed to this report from Moscow . CNN 's Richard Allen Greene reported from London .
Mikhail Prokhorov concedes he runs the risk of ending up in prison . The New Jersey Nets owner is worth $ 18 billion . Supporters of Russian Prime Minister Vladimir Putin march to back him . Tens of thousands demonstrate against parliamentary election results on Saturday .