Police in Nice are investigating the incident as is the national police oversight body (Representational)

A French policeman shot and killed a homeless Belgian man in the southern town of Menton after the man, who had tried to slit his wrists, lunged at an officer with a knife, investigators said.

The incident took place in a public park on Sunday evening.

Bystanders called the emergency services when they saw the man, who was in his fifties, trying to kill himself.

He had already made cuts in the veins of both arms as well as his throat and the back of his ankles when the police arrived on the scene, the prosecutor's office in the nearby city of Nice said.

The police tried in vain to get him to drop his knife.

When they failed, they tried to immobilise him with a stun gun but the man lunged at an officer with his knife, according to the prosecutor's office.

The officer's colleague then "used his service weapon and fired twice", killing the man, the investigators said, adding that the emergency services, who were at the scene, tried but failed to revive the victim.

Police in Nice are investigating the incident as is the national police oversight body, the IPGN.