The attack on a prison van in northern France that left two guards dead and three wounded began at 10:57 am (0857 GMT) and lasted just two minutes.

A black Peugeot SUV rammed the vehicle as it emerged, lights flashing and followed by a second prison service car, from the motorway toll gate at Incarville, just south of Normandy capital Rouen.

Out of the ramming car, "stolen a few days before" according to top Paris prosecutor Laure Beccuau, surged hooded attackers in black armed with military-style weapons.

The vehicle "had halted on the verge, waiting for the convoy" to pass the toll, Beccuau added, in a sign of an attack prepared down to the last detail.

CCTV images from the scene viewed by AFP show the beginning of the assault on the prison van, although a passing truck obscures some details.

More assailants can be seen arriving from behind the convoy, with prosecutors saying they jumped out of a white Audi car.

The prisoner, Mohamed Amra, was being escorted that day by five guards -- a "level three" protection that is the second-highest available for inmate transport in France, reserved for those implicated in terror or organised crime cases.

Guards were "of course armed", Beccuau said, adding that "initial findings from the scene lead us to think that some may have used their service weapons".

But the team had on them only "a basic Sig Sauer (automatic pistol) against military weapons," said Frederic Liakhoff, a representative for the FO union at the prison in the northern city of Caen where Amra was being transported.