The man turned up outside the entrance in mid-morning and the soldiers opened fire when he tried to force his way in with a knife in his hand, said first deputy prosecutor Gerard Nedeou.

"The victim was hit by two bullets and died from his wounds," he added.

On July 16, palace guards shot dead a man carrying a knife, who was subsequently described as mentally ill.

Social media seized on the incident, which came two days after Chadian guards shot at the tyres of a French army car parked near President Idriss Deby Itno's home in N'Djamena.

French gendarmes inside the car had been filming a monument opposite Deby's home "without realising the sensitivity of the area", Chad and the French embassy said in a joint statement.

France's anti-jihadist Barkhane force is based in N'Djamena and carries out operations across the Sahel region.

Last month, guards shot and wounded two French soldiers in front of the presidential palace.

Since seizing power in 1990 with French backing, Deby has survived several rebel attempts to overthrow him.

"The presidential guard are trigger-happy these days," one presidential adviser who asked not to be named told AFP.

"Those of us who work at the presidential palace are scared because these guards are not expected to know everybody who comes to work at the palace," he added.