In February last year, French magistrates led a two-week “raid” on an €80m (£68m) Paris mansion as part of a judicial investigation into the alleged “biens mal aquis” (ill-gotten gains) of the leaders of three African countries. This may have been the longest police raid in French history.

Two weeks were necessary, it was said, to make an inventory of the vast treasure-trove of property in the building, including jewellery, art, antique furniture and vintage wines. The magistrates also issued an international arrest warrant for Teodorin Obiang, 43, the son and heir apparent of President Teodoro Obiang of Equatorial Guinea.

The investigation – started in 2010 against the will of former President Nicolas Sarkozy’s government – has received renewed impetus and legitimacy in recent weeks from a landmark ruling by the Paris appeal court.

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Mr Obiang’s lawyers had asked for his “property” to be returned and the arrest warrant torn up. They had claimed that the building on Avenue Foch was part of the Equatorial Guinea embassy and therefore covered by diplomatic immunity. They also claimed immunity from prosecution for Mr Obiang on the grounds that his father had made him a vice-president of Equatorial Guinea after the international warrant was issued.

Both claims were dismissed. The judges said – in a ruling of potentially ground-breaking international importance – that diplomatic immunity could not be presumed to extend to systematic theft of state property.

Because of that ruling, the investigation has begun to bear fruit.

This week, a hundred wealthy collectors and dealers of luxury cars from across the world gathered for a spectacular auction in Paris.

On sale were two Bugattis, two Bentleys, a Rolls-Royce, a Ferrari, a Porsche, a Maserati and a Maybach. All had the same careful owner. All had low mileage or virtually no mileage.

The nine cars had seldom left their giant garage beneath the mansion near the Arc de Triomphe before they were seized by French police last year. They were sold this week by the Drouot auction house for €2.8m or an average of €311,000 each.

The “one careful owner”, of course, was Mr Obiang. Teodorin Obiang loves cars. This was only a small part of his fleet. According to French judicial documents, in November 2009 Mr Obiang imported 26 luxury cars from the US to France for $12m (£7.4m). They included seven Ferraris, four Mercedes-Benz, five Bentleys, four Rolls-Royces, two Bugattis, an Aston Martin, a Porsche, a Lamborghini and a Maserati.

Most of them were re-exported to Equatorial Guinea, whose 1,800 miles of roads are largely unpaved and, in rainy weather, negotiable only by four-wheel-drive. Nine of the cars remained in the garage below Mr Obiang’s seven storey mansion at 42 Avenue Foch, the exclusive Paris thoroughfare that is now largely foreign-owned. Mr Obiang – who denies having made his fortune by corrupt means – is appealing to France’s highest court, the Cour de Cassation, to get his property back.

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The French investigation extends to the alleged embezzlement of state funds on a heroic scale by the leaders of two other African nations: the President of Gabon, Ali Bongo Ondimba, and the President of Congo-Brazzaville, Denis Sassou Nguesso. The Paris appeal court ruling means that the two investigating magistrates, Roger Le Loire and René Grouman, can now proceed with renewed energy.

But to what end? How far can such an investigation go? Is it likely that any of the accused will ever appear in court in France? What happens to the money raised by sales of the seized property? Can it realistically be returned to the African people to whom it allegedly belongs?

Equatorial Guinea is one Africa’s smallest countries and, in theory, one of the most prosperous. Oil revenues make it the wealthiest single country in Africa per capita, but 70 per cent of the population lives beneath the United Nations poverty threshold of €2 a day. In Congo-Brazzaville (a former French colony to the north of the much larger Democratic Republic of Congo), three quarters of the population lives below the UN poverty line. In Gabon, the population is a little better off: only 20 per cent survive below the line.

William Bourdon is a French human rights lawyer who instigated the investigation in 2007 by making a formal complaint on behalf of the pressure group Transparency International France (TIF). The French state prosecution service refused to take up the complaint, generating accusations that former President Sarkozy was “protecting” the leaders of three former French colonies. In 2010, the Cour de Cassation overturned the state’s objections and, in effect, ordered it to launch a judicial investigation.

Mr Bourdon told The Independent: “I have had so many threats and insults and libel actions that I sometimes wish that I had never brought this case. But my spirits revive when I recall the hundreds and hundreds of messages that we have received from African people saying, ‘you have given us hope for the first time. Please continue, continue, continue’.”

Mr Bourdon agrees that it is unlikely Mr Obiang or any African leader will ever appear in a French court. But the property seizures and international arrest warrant – making foreign travel difficult – amount to a punishment in themselves. Beyond that, he hopes that the exposure of the extent of the arrogance and greed of the ruling clans will help to force political change.

The money raised from sales of seized assets is a conundrum, he admits. “The French courts must decide but it is obviously unacceptable that the money should be returned to governments who would just steal it again,” he says.

The former French possessions of the younger Mr Obiang are allegedly more than matched by the holdings of the Congolese and Gabonese first families (see graphic of major holdings in Paris). The family of President Bongo of Gabon is believed to own 39 luxury apartments or houses in the French capital. President Sassou Nguesso of Congo-Brazzaville has a modest 24 Parisian properties, according to documents leaked from the French investigation. Between them the two families are said to have 200 French bank accounts.

According to leaked French documents, President Bongo bought a Bentley Continental Flying Spur, worth €220,000, in France in 2009. Antoinette Nguesso, the president’s wife, bought a Mercedes E Class and his nephew, Wilfrid, a Porsche Panamera Turbo (each worth more than €100,000).

The French investigation has yet to uncover clear evidence that these possessions have been bought with cash that belongs properly to the two African states, and none of the properties has, so far, been raided or any items seized. Documents from the investigation leaked this week to the newspaper Le Parisien suggest, however, that the investigation into the Nguesso family is gathering pace. A French trading company called Franck Export, based near Orly airport, is alleged to have directed €9m of Congolese state cash towards personal acquisitions of the Nguesso family in France between 2005 and 2011. Franco Cantafio, the boss of Franck Export, has been placed under formal investigation for “complicity in the laundering and embezzlement of public money”.

In the case of Mr Obiang, the magistrates traced movements of money two years ago which suggested that he had in fact been pillaging public funds – hence the seizure of his mansion, cars and other property.

The younger Mr Obiang is evidently an art-lover as well as a car-lover. Tracfin, the French government’s anti-money laundering agency, has documented his purchase of more than €18m of objets d’art from the personal collection of the late fashion designer Yves Saint Laurent when they were auctioned in Paris in February 2009. The art is believed to have been distributed among his six homes abroad, including a seafront house in Malibu, California.

The bill for the 109 art works was settled by bank transfers from the Equatorial Guinea forestry board. At the time Mr Obiang was minister of agriculture and forests.

Of course corruption is not restricted to Africa, and the sleazy side of French public life has been exposed in recent months in a series of high-profile scandals. How does Mr Bourdon justify the use of stretched French judicial resources to investigate corruption abroad? “If we waited until we were above suspicion ourselves, nothing would ever be done,” he says.

“Obviously, France’s legitimacy as a champion of human rights and transparency might be greater than it is. But this investigation has been very important. A new anti-corruption wind is rising in France. Thanks in part to this investigation, a new wind is also rising in Africa.”

Wealth of nations: The target families

Teodoro Obiang:

The 71-year-old President of Equatorial Guinea - Africa’s longest-serving leader - came to power in a military coup which ousted his uncle in August 1979. The discovery of oil has made Equatorial Guinea nominally the wealthiest per head of population in sub-Saharan Africa, yet most of his people do not even have access to clean drinking water. The Obiang family had a Parisian mansion worth €80m, below, and other property worth up to €20m seized by French investigators last year.

Teodorin Nguema Obiang:

Cars ordered by the son of Equatorial Guinea’s President that were found in a garage at 42 Ave Foch were auctioned this week for €2.8m.

They were:

Bentley Arnage 2005

Rolls Royce Phantom 2005

Bentley Azur 2007

Ferrari 599 GTO 2010

Porsche Carrera 980 GT 2006

Bugatti Veyron 16.4e 2007

Maserati MC de 2005

Bugatti Veyron 16.4 Grand Sport 2010

Maybach 62 2004

Dennis Sassou Nguesso

The 70-year-old returned to power in Congo-Brazzaville after a civil war in 1997. He has since “won” two elections, both without any genuine, democratic opposition. Mr Nguesso claims to be influenced by socialism, but critics suggest he believes mostly in ‘socialism in one family’. When he attended the UN in 2006, his entourage occupied 44 rooms, running up a bill of €150,000. His family are believed to own 24 homes in Paris, including the two apartments below, worth €2.47m and €1.6m respectively.

Ali Bongo Ondimba

The Gabon President, 54, succeeded his father Omar Bongo in October 2009. He was chosen as a candidate by the dominant party in dubious circumstances. Reports at the time suggested that he was seen as an upstart – he was described as ‘a spoilt child, born in Congo-Brazzaville, brought up in France, hardly able to speak indigenous languages and with the appearance of a hip-hop star’. His family are thought to own 39 Parisian homes. Those pictured are worth €18.9m and a whopping €100m respectively.