The incident took place at about 9am local time.
At least one person has died in the attack and several others have been injured, according to an initial assessment.
The suspect was promptly arrested by police.
Witnesses on social media have reported hearing shots fired as armed police search the area.
People have been evacuated from the scene for safety.
Gérald Darmainin, France’s interior minister, has said he is chairing a crisis meeting into the incident.
He wrote on Twitter: “#Nice : a police operation is in progress.
“Avoid the area and follow the instructions. “After having had the mayor of Nice @cestrosi, I am chairing a crisis meeting at the Ministry of the Interior.”
More to follow…