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French security forces are ready to smother the centre of Paris with a 'last resort' chemical weapon in a bid to keep rioters away from key buildings, it has emerged.

Astonishing revelations about the 'debilitating powder' – which can be spread across an area the size of six football pitches in just 10 seconds – highlights the increasing desperation of President Emmanuel Macron's administration as it faces up to a law and order crisis.

Paris has been hit by five straight weeks of violence by the Yellow Vests protest movement that has seen national monuments including the Arc de Triomphe ransacked.

There were 168 arrests in the city on Saturday alone as the demonstrators – who are named after their high visibility jackets – fought running battles with police, who responded with water cannon, baton charges and teargas.

(Image: ETIENNE LAURANT/EPA-EFE/REX/Shutterstock) (Image: REUTERS)

Now senior officers have confirmed that some of the 14 armoured cars deployed by gendarmes contained "a radical device that was only to be used as a last resort" against their own citizens.

A gun-like distributor on the vehicles' turrets can spray the powder over 40,000 square meters in 10 seconds, Marianne magazine reports.

The high-density noxious product contains the same power as 200 teargas grenades, and is designed to knock people out indiscriminately in an emergency.

"If a large crowd forced barriers through the security perimeter, then the powder would be used as a last resort in order to stop them," said a source at the Paris police prefecture.

(Image: REUTERS)

But it is sure to raise concerned questions among civic rights groups, as well as monitoring organisations, including the Hague-based Organisation for the Prohibition of Chemical Weapons, of which France is a member.

Colonel Richard Carminache, of the Gendarmerie, confirmed that the controversial devices had "never been used in cities to my knowledge".

Each distribution would result in "a highly concentrated teargas cloud, the equivalent of 200 grenades in one go," said Col. Carminache, who added: "It’s best to run to get out."

(Image: AFP/Getty Images)

Teargas is classed as a chemical weapon, and is actually banned from warzones, in line with international agreements.

Yet French gendarmes and police – who have been criticised during the latest law and order crisis for acts of extreme violence against civilians – use it constantly.

In Britain, teargas use is heavily restricted, and never used indiscriminately against large crowds containing men, women and children, as happens in France.