Simultaneous attacks on multiple targets by gunmen and suicide bombers working in unison: a nightmare scenario that France's anti-terror agencies had dreaded for months came true in Paris on Friday night.

Security officials and experts have predicted that an unprecedented attack was in the offing, and would be nigh impossible to thwart.

"Determined guys who are prepared to die, who have studied their target and have a solid operational background, they can do a lot of damage," Yves Trotignon, who used to work for DGSE intelligence agency's anti-terrorist service, recently told AFP.

"More jihadist fighters are graduating as veterans every day. Faced with that, it has to be said, the (security) services are overwhelmed," he said.

The events of Friday were exactly the kind of multi-pronged attack authorities had been dreading.

A body lies outside the concert hall. Photo: AFP, Reuters

So far this year they have been fortunate: more than one potential bloodbath was prevented by the offender's own ineptitude.

In April, Algerian IT student Sid Ahmed Ghlam was arrested after he shot himself in the leg by accident, leading police to uncover a plot to attack a church in Paris' Villejuif suburb. And in August, two off-duty US servicemen and a friend overpowered a gunman who opened fire on passengers on a high-speed train from Amsterdam to Paris.

But the authorities' luck was bound to run out as radicalised Muslims return from warzones battle-hardened and well-trained, said the experts.

And they are coming home in such great numbers that it is impossible to keep tabs on them all.

Since the January slaying of 17 people in joint attacks on Charlie Hebdo, known for its satirical caricatures of Islam, and a Jewish supermarket elsewhere in Paris, anti-terror, intelligence, police and rescue services have been rehearsing for another such multi-pronged assault.

Like anti-terror services the world over, French experts closely studied the Mumbai attacks in November 2008, in which 10 attackers struck five different spots in concert, killing about 170 people. But plan as they like, the experts agree there will always be an element of surprise. And onFriday, the whole was shocked.

And things have become complicated by wall-to-wall media coverage -- with attackers nowadays positioning themselves "for maximum publicity", a senior anti-terror official recently told AFP on condition of anonymity.

"Right now, we fear kalashnikov attacks" -- exactly like the one on the Bataclan theatre which ended when police stormed the building, but only after at least 82 people were killed.