A 'guru' who is accused of cheating three generations of a wealthy and aristocratic French family out of their fortune went on trial on Monday .

For a decade Thierry Tilly brainwashed 11 members of the de Védrines family into believing that their lives were threatened by a secret masonic plot that only he could save them from, say investigators.

After persuading them to barricade themselves first into their ancestral chateau then within a suburban house in Oxford and employing what police described as "acts of torture and barbarism", he is accused of tricking the family into handing over up to €4.5m (£3.6m).

At the Palais de Justice in Bordeaux on Monday, Tilly, 48, went on trial for "fraud, abuse of weakness, violence against the vulnerable, and holding people against their will for up to seven days".

The story of the de Védrines and Thierry Tilly is as mysterious and convoluted as the plot of a Dan Brown novel, to which it is has frequently been likened. Investigators and lawyers are at a loss to explain how the small, bespectacled law school dropout and failed businessman with a conviction for fraud, convinced the de Védrines to part with their home, along with the family silver and their entire fortune.

The de Védrines' matriarch Guillemette, who died in 2010 aged 97, and her children Philippe, 64, an executive with Shell Oil, Charles-Henri, 54, a gynaecologist and local politician, and their sister Ghislaine, 56, the director of a secretarial school in Paris, were, said friends, proud of their status as protestant nobles but were unlikely dupes. It was Ghislaine who introduced Tilly, who worked as an administrator at her school, to the family in the late 1990s. He claimed to be a descendent of the Austro-Hungarian Habsburg nobility and said his communist grandmother had held "salons" with former socialist president François Mitterrand.

Investigators say it was Tilly's claims of connections in high places and money-making schemes that first attracted the de Védrines to him. By 2001, after Tilly convinced family members they were the targets of a masonic plot, they had barricaded themselves in their ancestral home, the turreted manor house Château Martel near Monflanquin, a pretty medieval village in the Lot-et-Garonne.

As well as Guillemette and her children, the group included Charles-Henri and his wife Christine's children, Guillaume, now 24, Amaury, 22, Diana, 17, and Ghislaine's two children, Guillemette, 25 and François, 23.

When Ghislaine's husband, Jean Marchand, a business journalist, expressed his scepticism of Tilly, he was banished and accused by his wife and brothers-in-law of belonging to a "network of evil".

In an article in Vanity Fair in 2010, Marchand described Tilly as a sort of "brain burglar". "He opened their heads, took out their brains and put in a new one."

After the de Védrines, who had increasingly withdrawn from the world, stopped paying their taxes the French fiscal authorities seized Château Martel's furnishings and auctioned them off. In 2007, Tilly took them to Oxford, where they lived in a succession of anonymous suburban homes, some of which still have rent outstanding.

Tilly was arrested in Switzerland in 2009 after Christine de Védrines told police she had been violently abused by her family and Tilly because she had failed to provide the "key" to the fortune they had convinced themselves she possessed.

At the Palais de Justice in Bordeaux on Monday, Alexandre Novion, the lawyer for Tilly, who has denied all the charges, said he was "confident". He rejected suggestions his client was a "guru", claiming the affair was "a power game".

After seeing Tilly, a small man in a black polo neck and frameless glasses in the dock on Monday, Ghislaine de Védrines told journalists outside the court in Bordeaux: "He's a liar and a storyteller." She added: "I know very well how he tries to get the upper hand, except here he's in front of professionals, which wasn't the case when he was with us. He kidnapped us, telling one of us one thing and another something else, and set us against one another."

Marchand, who alerted the authorities, said that he hoped the trial would be a warning to others in a similar situation. "That man has made a career out of mental manipulation," he said.

A second man, Jacques Gonzales, 65, linked to Tilly, who has also denied charges of fraud and abuse of weakness, is also on trial.

The trial continues.