Ivanishvili, whose fortune is estimated at $5.5 billion by U.S. Forbes magazine, declared his ambitions in an open letter last week and called on President Mikheil Saakashvili, the pro-western leader of Georgia's 2003 'rose' revolution, to resign.

The reclusive businessman, also known by the Russian first name of Boris and whose banking-to-retail interests are mainly in Russia, was quickly denounced by Tbilisi as a "Kremlin stooge" and stripped of his Georgian citizenship.

But, in his first media interview since 2005, Ivanishvili told Reuters at the weekend he would rally the "healthy" elements of Georgia's divided opposition and oust Saakashvili's ruling party in parliamentary elections due in autumn next year.

"With a probability of at least 90 percent we will enter parliament with an absolute majority," said Ivanishvili, 55.

"It's too early to say whether we will create a bloc or it will be one party," he said. "Consultations with some of the healthy opposition forces in Georgia are under way."

Ivanishvili poses a challenge to the pro-western course taken by Saakashvili in eight years as leader of Georgia, a nation of 4.7 million on Russia's restive southern flank that is a strategic corridor for exports of Caspian oil to Europe.

MYSTERY MAN

Long an enigmatic figure even in his own country, Ivanishvili is a slight man with hooded green eyes and brown hair touched with grey. He wore a black Italian suit and matching Rado watch.

The passionate art collector gave the interview in his $50 million hilltop headquarters, which has been described as a fortress by its Japanese architect, Shin Takamatsu, and invites comparisons with U.S. media mogul William Randolph Hearst's castle retreat in California.

Works by Damien Hirst and Roy Liechtenstein hang in the steel-and-glass halls of the business and hospitality center, whose courtyard is dotted with sculptures by Zaha Hadid, Anish Kapoor and Henry Moore.

Ivanishvili said he had made the decision to enter politics after a week of "agonizing doubts," having been a "hostage" of his past vows to stay out of public life.

"I was hesitating until the very end," he said. "But now I think my position will strengthen day by day, while their position will weaken," he said of Saakashvili's government.

"They tried to exclude me from the political process, but I'm not going to stop," he said of a ruling that his Georgian citizenship had automatically become invalid when he acquired a French passport in 2010.

"If they bar me from participating in this election, it won't be legitimate," added Ivanishvili, who also has Russian citizenship. Georgia does not allow multiple citizenship.

SELLING OUT

Ivanishvili, who founded Rossiysky Kredit Bank in 1990 with partner Vitaly Malkin and owns a chain of Russian pharmacies and real estate interests in Moscow, said he was ready to sell his assets to focus on politics.

"The process of appraising my assets has already begun," he said. "I realize that I won't be able to sell them for the maximum price, but I'm ready."

Ivanishvili said his first priorities after winning power, as speaker of parliament or prime minister, would be to launch constitutional and judicial reforms, cut business red tape and protect media freedoms.

Georgia approved a constitutional reform last year, changing the ex-Soviet republic's presidential system to a "mixed" one with a more powerful premier and parliament from 2013, when Saakashvili's second term allowed by the constitution expires.

Saakashvili's opponents say the 43-year-old leader, elected after the bloodless revolution of 2003, plans to become prime minister at the head of his United National Movement, which has a large majority in the current parliament.

"I know that he (Saakashvili) wants to stay in power," Ivanishvili said. "We should not allow that."

He said two or three years would be enough to implement his radical agenda, after which he would leave politics.

"I hope to astonish Europe with the level of democracy that I will create in Georgia," he said. "It will be such a real democracy that even Europeans will want to invest in Georgia."

Critics accuse Saakashvili of curbing freedoms and leading Georgia into the war with Russia in August 2008. Georgian forces were routed in five days and Moscow went on to recognize breakaway South Ossetia and Abkhazia as independent states.

Ivanishvili accused Russia of "unheard-of aggression" against Georgia, but said Saakashvili's reckless foreign policy had provoked the conflict.

He said Georgia, which has a de facto veto over Russia's long-running bid to join the World Trade Organization, should find a role that is "comfortable for the United States and Europe, and acceptable for Russia."

Russia was not "the worst example of an undemocratic state," he said, while Prime Minister Vladimir Putin, who has announced a plan to return as president, was a genuinely popular leader.

"The Russian people like this man. It's their business, their choice, although a more democratic Russia would be more interesting for Europe and for the United States," he said.

"It's very likely that the next Russian government will launch a fight against corruption, will seek a rapprochement with the West and implement real democratic reforms."

(Reporting by Margarita Antidze; Editing by Douglas Busvine and Tim Pearce)