Russian metals tycoon and US basketball team owner Mikhail Prokhorov speaking in Moscow on Monday.CNN

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.

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.

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 didn'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 isn't an individual person and doesn'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 wouldn'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.