PARIS -- France's vigilance against terrorism is raised to the highest level of "attack emergency" throughout the national territory following the knife attack in Nice, Prime Minister Jean Castex announced on Thursday.

Speaking at the National Assembly, Castex denounced the knife attack in Nice as cowardly and barbaric and specified that a defense council meeting would take place on Friday morning.

Early Thursday morning, a knife attacker stabbed to death three people at Notre-Dame basilica in central Nice. The alleged perpetrator as shot by police and transferred to hospital.

France's national anti-terrorism prosecutor's office (PNAT) said it would investigate the incident under "attempted assassination in connection with a terrorist enterprise."

Following the waves of attacks in 2015 and 2016, France has made the "Vigipirate plan" a permanent instrument in the fight against terrorism.

The plan has three levels of alerts: "vigilance" which represents a permanent security posture; "enhanced security - attack risk", which responds to a high terrorist threat; and "attack emergency", which responds to an attack or an immediate threat of a terrorist attack.

The attack emergency level can be set up immediately following an attack or if an identified and non-localized terrorist group comes into action. When activated, the plan makes it possible to ensure the exceptional mobilization of resources, and to disseminate information to protect citizens in a crisis situation, according to the government website.