The United States stands with France, President Donald Trump said in a message of solidarity after his French counterpart Emmanuel Macron declared the country was 'under attack' following another deadly extremist rampage.
A man armed with knives killed three people at a church in Nice in the first of four terror attacks that unfolded in four hours on Thursday in Avignon, Lyon and Paris.
The attacks were carried out by non-French nationals who had come to the country from Tunisia, Pakistan and Afghanistan.
"Our hearts are with the people of France," Mr Trump tweeted after the Nice attack. "America stands with our oldest ally in this fight. These radical Islamic terrorist attacks must stop immediately. No country, France or otherwise can long put up with it!"
The Tunisian man arrested in the Nice attack only arrived in France this month from the Italian island of Lampedusa, an official told AFP.
He called himself "Brahim" when he was arrested and later claimed to be Brahim Aouissaoui, the source said.
Mr Macron vowed to double the number of soldiers guarding key landmarks as the country’s terror alert system was raised to its highest level.
At 9am Aouissaoui, illegally in the country and armed with a knife, entered the Notre-Dame church in Nice and killed three people: church warden Vincent Loques and two women.
The knifeman tried to behead two of them and severely wounded the other woman, who fled to a nearby cafe and later died. Police shot the assailant and took him into custody.
Just hours later, a man shouting “Allahu Akbar” was shot dead by police after threatening passers-by with a handgun in Montfavet, near the southern French city of Avignon.
Half an hour later police arrested a suspected terrorist, of Afghan origin, in Lyon after he tried to board a train armed with a 30-centimetre knife.
At 1pm police foiled a fourth attack at Saint-Martin Church in the Paris suburb of Sartrouville, where a man was armed with knife.
A security guard was also stabbed at the French consulate in the Saudi Arabian city of Jeddah.
Nathalie Goulet, former president of the Senate commission of inquiry into extremist networks, told The National that the suspects were not on France's intelligence watch list.
“We need to try and improve our intelligence because it is almost impossible to stop this kind of unprecedented attack,” Ms Goulet said.
“France has been under attack today in Islamist incidents carried out by individuals who were not French nationals.
"There were not on our File S list. In Nice, the victims were slaughtered, almost beheaded. We need to take urgent action now to prevent more atrocities.”
Hours after the attacks, former Malaysian prime minister Dr Mahathir Mohamad risked inciting further violence after saying Muslims "have a right to be angry and to kill millions of French people".
Speaking from Nice’s Notre-Dame church, Mr Macron promised that France would stand firm and “never give up” on its values.
“France is under attack by ... terrorists," he said. "We are attacked over our values, for our taste for freedom, for the ability on our soil to have freedom of belief.
"And I say it with lots of clarity again today: we will not give any ground."
Police and anti-terror prosecutors have launched an immediate investigation.
It comes just weeks after the beheading of French teacher Samuel Paty, and Nice Mayor Christian Estrosi said it was an attempted copycat killing.
"The methods match, without doubt, those used against the brave teacher in Conflans-Sainte-Honorine, Samuel Paty," Mr Estrosi said.
Mr Loques, a father of two and lay member of staff responsible for maintaining the basilica, had just opened the church for the day when the attack happened.
The Tunisian slit his throat, partially beheaded an elderly woman and badly wounded a second woman.
Mr Loques and the elderly woman died on the spot, while the other woman made it out of the church into the nearby cafe, but she died from her wounds, Mr Estrosi said.
“Thoughts for Vincent Loques, sacristan of the Notre-Dame basilica in Nice, cowardly murdered by a terrorist,” politician Eric Ciotti said.
“He was extremely devoted to his church, I am thinking of his family, his relatives and the whole Catholic community.”
Father Gil Florini, a Roman Catholic priest in Nice, described Mr Loques as a “very kind person".
Mr Estrosi said the suspect was shot by police but was alive and receiving medical attention in hospital.
The mayor said he kept repeating "Allahu Akbar" even after his arrest, and “the meaning of his gesture left no doubt".
"Enough is enough," Mr Estrosi said.
'Run, run, somebody is stabbing people'
Father Philippe Asso, who serves at the basilica, said Mass was not under way at the time of the attack but "people come in to pray at all hours".
Daniel Conilh, 32, a waiter at a nearby cafe, said it was just before 9am when "shots were fired and everybody took off running".
"A woman came in straight from the church and said, 'Run, run, someone has been stabbing people'," he said.
Prime Minister Jean Castex said the country's terror alert system was raised to "emergency", the highest level, after the third apparent extremist attack in just over a month.
The Catholic Church in France described the Nice attack as an "unspeakable act".
"Christians must not become a symbol to be destroyed," the Conference of Bishops of France said.
The French Council for the Muslim Faith also condemned the attack.
"As a sign of mourning and solidarity with the victims and their loved ones, I call on all Muslims in France to cancel all the celebrations of the holiday of Mawlid."
The holiday marks the birthday of the Prophet Mohammad, celebrated on Thursday.
In 2016, a gunman drove a heavy truck into a crowd celebrating Bastille Day in Nice, killing 86 people and injuring scores in an attack claimed by ISIS.
The latest attack came while France was still reeling from the beheading of Mr Paty carried out by a man of Chechen origin.
The attacker, Abdoulakh Anzorov, had said he wanted to punish Mr Paty for showing pupils cartoons of the Prophet Mohammed published in satirical magazine Charlie Hebdo.
It was not clear what the motive for the Nice attack was, or if it was connected to the cartoons, which Muslims consider to be blasphemous.
Since Mr Paty's killing, French officials have reasserted the right to display the cartoons and the images have been shown at marches staged in solidarity with the dead teacher.
That has prompted an outpouring of anger in parts of the Muslim world, with some governments accusing Mr Macron of pursuing an anti-Islam agenda.
In a comment on recent beheadings in France, Moscow said on Thursday it was unacceptable to kill people, but also wrong to insult the feelings of religious believers.
World reacts to 'brutal, barbaric' attack
World leaders condemned the deadly stabbings and expressed solidarity with France.
Besides the US president, condemnation came from the Pope and European leaders, and from Arab and Muslim countries.
The Ministry of Foreign Affairs and International Co-operation said the UAE expressed its strong condemnation of these criminal acts.
The ministry stressed its “rejection of all forms of violence that aim to destabilise security and stability and are inconsistent with religious and humanitarian values and principles”.
Lebanese Prime Minister-designate Saad Hariri posted on Twitter “the strongest condemnation” of the attack in Notre-Dame Church in Nice, calling it a heinous crime.
“Terrorism has no religion and all Muslims are called on to reject this criminal act that is not related to Islam or to the prophet of love,” he wrote.
Saudi Arabia "strongly condemns" the attack, the kingdom's Foreign Ministry said.
"The kingdom categorically rejects such extremist acts, which contravene all religions, while stressing the importance of avoiding all practices that generate hatred, violence and extremism," it said.
Jordan’s Foreign Minister, Ayman Safadi, said: “We condemn the terrorist crime.
"Terrorism is a common enemy that has nothing to do with our joint human values and the great Islam and its values of peace and sanctity of life.
"We must all fight culture of hate that feeds it in all its forms and promotes respect for the other.”
The Muslim Council of Elders voiced its condemnation, while saying that it “affirms that the recent upsurge in terror and hate attacks requires serious action by countering extremism and criminalising hate speech, which may fuel further attacks and other acts of hate”.
Turkey, which is involved in a heated diplomatic dispute with Paris over offensive cartoons, added to the condemnation.
"We strongly condemn the attack committed today inside the Notre-Dame church in Nice," the Turkish Foreign Ministry said.
The UN Secretary General, Antonio Guterres, called it a “heinous attack”.
UK Prime Minister Boris Johnson said he was "appalled" to hear of the attack.
Chancellor Angela Merkel said Germany stood with France after the "brutal" killings, while Italian Prime Minister Giuseppe Conte said the "vile attack" would not "shake the common front defending the values of freedom and peace".
The EU slammed the attack.
"I condemn the odious and brutal attack that has just taken place in Nice and I am with France with all my heart," European Commission president Ursula von der Leyen tweeted.
"My thoughts are with the victims of this hateful act. All of Europe is in solidarity with France. We will remain united and determined in the face of barbarity and fanaticism."