A total of 147 people were murdered by terrorists in the capital during atrocities in January and November.
Many of those responsible were French passport holders with North African backgrounds, leading to far-Right politicians calling for a massive clampdown on immigration.
But Mr Calvar, 60, warned there is evidence that radical Right-wing French groups have been massing arms in preparation for their own attacks on mosques and synagogues.
‘I think we will win against terrorism,’ Mr Calvar said, but predicted the ‘confrontation between the extreme Right and the Muslim world’, adding: ‘We’re on the verge of a civil war. I think this confrontation is going to happen. One or two more attacks and it will take place. It is up to us to anticipate and stop all those groups who would trigger clashes.’
The closed inquiry was held on May 24, but Mr Calvar’s dramatic comments were leaked to French media yesterday. He said: ‘Where is the spark going to come from that will light the powder, transforming France into an uncontrollable country where groups take up arms and hand out their own justice? Who sees a crumbling country where violence and vengeance alternates between two camps, where the spiral of attacks does not stop?
‘Nothing is excluded in a country which is already as eruptive as France today.’
Mr Calvar said he particularly feared ‘punitive expeditions in the suburbs’ to make immigrants pay collectively for alleged crimes by a minority. Referring to the mass sex attacks in Germany on New Year’s Eve, Mr Calvar said French Right-wingers could hand out summary justice for the kind of ‘gang rapes committed by North African immigrants in Cologne’.
There have been no convictions for rape following the Cologne incidents, with German police saying CCTV evidence was not good enough to charge suspects.
Mr Calvar also said his nightmare scenario could just as easily be applied to European neighbours, including Britain. ‘It’s this that worries me when I talk to all the European colleagues,’ he said. ‘We will, at one time or another, have to generate resources to deal with other extremist groups, because the confrontation is inevitable.’
Mr Calvar said all extremist groups – Left, Right and Islamist – know that there is enough weakness in the European security apparatus for a small group to ‘cause maximum chaos’.
He specifically called for more money to be put into armaments and other resources for the security forces, and for greater legal powers to carry out surveillance and arrest suspects.
He also said he is ‘convinced’ that Islamic State will change its tactics in France, and increase its firepower, which until now has mainly relied on outdated AK-47 rifles and suicide belts.
He said he feared a move towards car bombs and more conventional explosive devices, allowing terrorists to attack without risking their own lives, and that he believed Islamist extremists will look to use booby-trapped cars in the future.
He said: ‘I’m convinced they’ll go to booby-trapped vehicles and bombs, thus upping their power.
‘We know very well they’re going to use this mode of operating.
‘They’re going to end up sending commandos whose mission is to organise terrorist campaigns without necessarily going to the assault with death awaiting them.’
He also raised the possibility of extremists using ‘dirty bombs’ and the natural poison ricin, saying several radical groups had studied the toxin in the past.
The Armed Islamic Group, which caused terror in Algeria in the early 1990s, was looking to put the substance on car door handles to create a panic effect, Mr Calvar said, and this tactic was also studied in northern Iraq and in the remote Pankisi Valley in Georgia, once a stronghold of Chechen militants.
Earlier this week a leaked report revealed a staggering 1,200 women were sexually abused in German cities during the New Year’s Eve celebrations.
The police document said detectives believe 2,000 men were involved across various cities but that the bulk of the crimes were committed in Cologne and Hamburg, where 600 and 400 sexual assaults on women were reported respectively.
Of the 2,000 perpetrators, only 120 have been identified, and about half of them were foreign nationals who had only recently arrived in Germany.
Eyewitnesses described the night of December 31, which saw many Arab and North African asylum seekers and immigrants gang rape and attack hundreds of women, as being akin to a war zone.
The warning from Mr Calvar follows similar concerns expressed by prominent police, army and security experts from across Europe.
In May the former head of MI6, Richard Dearlove, said Europe would face a ‘populist uprising’ if its governments did not take control of the migrant crisis.
German Chancellor Angela Merkel was also told by security experts last October that middle class citizens in Germany were becoming ‘radicalised’ because of her open borders migrant policy.
She was warned that it could lead to widespread disorder.