They were preparing for a wedding in Chorvila last week. But, then, they are always preparing for weddings in this village of a few hundred, some two hours' drive west of Georgia's capital, Tbilisi. With Chorvila's modernist wedding centre providing a welcome, albeit incongruous, addition to the village's poorly constructed houses, couples are assured a memorable place in which to hold their nuptials. They are also guaranteed a $3,200 donation from Chorvila's most famous son, Bidzina Ivanishvili, who paid for the centre to be built and throws in the wedding for free.

Until this month, few people in Georgia – and even fewer outside – knew much about Ivanishvili. The 55-year-old businessman whom Forbes magazine estimates is worth $5.5bn, never gave interviews and guarded his privacy tightly, living in a 30,000sq ft compound high on a hill above Tbilisi.

Such is Ivanishvili's secretive nature that when the Observer was flown to Georgia to interview him even one of his advisers conceded: "People may know his name but they can't pick him out of a lineup." But this will change next Sunday, when Ivanishvili launches his Georgian Dream alliance, a new political movement intent on ousting the incumbent UNM party and its charismatic president, Mikheil Saakashvili.

Once a supporter of Saakashvili, Ivanishvili, who made his money in Russia in banking, retail and property, says he has grown disillusioned with his country's direction. His entry into politics, he insists, is reluctant, temporary and necessary. "It was not my wish to come into politics. I was not a public person; I preferred to spend my birthdays with family and friends. But the 2008 elections were fraudulent so I decided to finance the opposition to make them stronger. There is a nihilism ingrained in [Georgian] people. With my appearance on the stage, people have hope."

Ivanishvili does little to hide his disdain for Saakashvili, whose second and final term in office expires in 2013, after which there is speculation that he may "do a Putin" and run for prime minister.

Ivanishvili's foray into politics is not without personal risk. The thick-set security guards who patrol his compound – which boasts a helipad and sculptures by Anish Kapoor and Robert Indiana – are testimony to the father of four's heightened vulnerability. His Georgian citizenship has been revoked; a putative bid to buy a television station was blocked; a security van carrying cash from the Cartu Bank that he owns was impounded; a bill to strip him of the bank has been introduced before parliament.

In the past month, ministers have painted Ivanishvili, who moved back from Moscow eight years ago, as a Russian spy. Cynics say it is inconceivable that he could have acquired his fortune without Kremlin backing and that now Putin is calling in favours. Coming only three years after Russia invaded Georgia, this charge is toxic.

Ivanishvili who names former Czech president Václav Havel as his hero, bristles at the suggestion. "I've never been an oligarch. The definition of an oligarch is someone who has co-operated with the government and I never did."

Irakli Alasania, Georgia's ambassador to the UN during the war and now leader of the Free Democrats, which, with the Republicans, comprises the two parties united under the Georgian Dream umbrella, describes the Russian spy allegations as "horseshit". Ivanishvili has pledged to sell his Russian interests. A 1% stake in Gazprom has been pared down and the remaining shares will be sold, he insists. But Ivanishvili does appear more conciliatory towards Russia. While talking up Georgia's desire to join the EU and Nato, he acknowledges that "an ideal situation would be to form an optimal relationship with Russia."

The size of Ivanishvili's resources is evident in Sachkhere, a town of 48,000 people a few miles from Chorvila where his businesses have created 2,500 jobs and where he is a hero to his employees. The hospital under construction; the Sameba Cathedral, the largest church in the country; the police station on the road outside; the mountain troops' barracks on its outskirts; its gleaming glass-housed public swimming pool; the water treatment plant; the region's 30 or so schools; the cinema – all paid for by Ivanishvili, who over the past five years has given more than $1bn to a country he left when a teenager. His largesse is linked to geography. In Chorvila, Ivanishvili even pays his former neighbours' gas and electricity bills and ensures that their homes have fridges and washing machines.

But it also fans out across Georgia's incipient civil society. Recently it emerged that Ivanishvili was paying the pensions of some 3,000 artists, sports stars, writers and scientists. Patronage on this scale prompts accusations that the billionaire wants to buy his country's favour. A more damaging charge is that Ivanishvili will not be able to make the leap from philanthropist to political leader. "You can't run a country as a charity," acknowledged Andrew Wilson, senior policy fellow at Wider Europe, an organisation that analyses the EU's relations with its neighbours.

Ivanishvili's plan, he claims, is to become prime minister for only two or three years, enough to implement constitutional reforms that will enhance Georgia's democratic institutions, after which he will stand down and, quixotically, fund opposition parties to keep the new government in check.

On the walls of Ivanishvili's capacious office are paintings by De Kooning, Freud and Monet. They are replicas: the real artworks in his $1bn collection are locked away for insurance purposes.

Intriguingly, however, it is rumoured that the odd one is genuine. It was difficult to determine the real from the fake.