The incident took place inside the church at about 9am local time. At least three people have died in the attack and several others have been injured. The incident is believed to be terror-related, according to the city’s mayor.
Two of the victims, a man and a woman, are understood to have been decapitated, according to a police spokesperson.
According to the mayor of Nice, Christian Estrosi, the security guard of the church is one of the victims.
The 23-year-old suspect was promptly arrested by police but was injured, so has been taken to a nearby hospital.
A security perimeter has been set up around the church, located in the city centre.
Local media report police are searching for a potential accomplice.
They are asking residents to be careful when entering the city centre.
A stabbing has taken place in Nice this morning
Nice attack: The Mayor believes the incident is terror-related
David Olivier Reverdy, representative of the Police Union, said: “The man allegedly attacked two women and a man”.
“The municipal police were the first on the scene. The man was shot and taken to hospital. For the moment, we do not know if he acted alone.”
Pictures from the scene show the area around the Notre Dame almost completely empty, except for armed police.
Witnesses on social media have reported hearing shots fired as armed police search the area.
One person described the incident as “very distressing”, after hearing about seven shots fired and seeing police officers storm the church.
National Police have urged people to avoid the area and said there is no need to panic.
The Mayor of Nice has said he suspects the incident is terror-related.
Nice attack: Police have cordoned off the area surrounding the church
He wrote on Twitter: “I am on site with the @PoliceNat06 and the @pmdenice who arrested the perpetrator of the attack.
“I confirm that everything suggests a terrorist attack in the Basilica of Notre-Dame de #Nice06.”
Speaking about an hour after the incident, Mr Estrosi told reporters: ““My first words are compassion for the relatives and families of the victims and the parish community.
“And all the people of Nice who are in shock. Nice like France is paying a great price as a victim of Islamo-fascism.
“The attacker did not stop repeating ‘Allahu akbar’ while he was being treated by paramedics in front of us. Enough is enough.”
Nice attack: At least three people have died
Nice attack: A crisis unit has been formed and will be joined by Emmanuel Macron and the prime minister
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.”
The Prime Minister will also join the crisis unit, having hastily left the National Assembly where he was defending the country’s new lockdown measures.
French President Emmanuel Macron will also go to Nice.
French members of the elite tactical police unit RAID leave after searching the Basilica
The President of the Republic will first make a visit to the crisis center on Place Beauvau.
An investigation has been opened for “assassination in connection with a terrorist enterprise”, “attempted assassination in connection with a terrorist enterprise” and “criminal terrorist association”.
The probe has been entrusted to the General Directorate of Internal Security and the Central Directorate of the Judicial Police.
Boris Johnson has said he is “appalled” by the Nice attack.
He wrote on Twitter, in both French and English: “I am appalled to hear the news from Nice this morning of a barbaric attack at the Notre-Dame Basilica.
“Our thoughts are with the victims and their families, and the UK stands steadfastly with France against terror and intolerance.”
Nice attack: The Mayor went to the scene of the incident
Two hours after the attack in Nice, a man armed with a knife in Avignon allegedly tried to attack police in the street.
He was killed by the police.
It is not yet clear whether the two attacks are linked.
The two incidents come less than two weeks after history teacher Samuel Paty, 47, was beheaded outside his high school after showing his class caricatures including one of the Prophet Muhammad during a discussion on free speech.
Additional reporting by Maria Ortega