France stepped up security nationwide against new Islamist attacks a day after the fatal stabbings at a church in Nice.
Police overpowered a man in the French capital Paris on Friday (October 30) when he threatened officers with two knives.
President Emmanuel Macron has deployed thousands of soldiers to protect sites such as places of worship and schools.
France's security alert is now raised to its highest level.
Police were holding a 21-year-old Tunisian migrant, named as Brahim al-Aouissaoui.
He arrived in Nice by train from Italy on Thursday morning and made his way to the Notre Dame church.
French police believe he was the attacker who killed a 55-year-old sexton and beheaded a 60-year-old woman, before stabbing a mother-of-three, Simone Barreto Silva.
Who fled to a nearby cafe and raised the alarm before she died.
In the Tunisian city of Sfax, Aouissaoui's family told Reuters they were shocked at the idea he had committed such a violent crime.
This is his sister:
"No one can understand how this happened."
His brother said the family had spoken to him on a video call outside the church hours before the attack.
He had shown no sign that he planned any violence.
Thursday's attack, on the birthday of the Prophet Mohammad, took place at a time of swelling Muslim anger across the globe.
At the issue of French cartoons depicting the prophet, which they deem insulting and blasphemous.
Tens of thousands of Muslims protested across the globe, chanting slogans and accusing Macron of Islamophobia on Friday (October 30), for defending the right to publish such cartoons.
France was already in mourning from another deadly attack -- when a school teacher was beheaded in a Paris suburb. He'd shown his pupils similar cartoons in a class on freedom of expression.