French officials have identified one of the Paris terror attackers as they detained members of his family for questioning.
A judicial official said that 29-year-old Frenchman Ismael Mostefai had been flagged for links to Islamic radicalism.
Mostefai’s father, a brother and other family members have been detained and are being questioned.
The mayor of the French city of Chartres, Jean-Pierre Gorges, identified Mostefai as a resident in a Facebook post.
The Paris prosecutor said one of the attackers was a 29-year-old Frenchman born in the Chartres region who had been known to authorities for radicalism.
The prosecutor said he was identified by fingerprints on a finger found in the carnage of the Paris attacks Friday night, which left at least 129 dead and hundreds wounded.
The Islamic State (IS) group has claimed responsibility for the attacks.
The arrests came as the authorities stepped up their hunt for accomplices of the extremists responsible for a gun and bomb rampage targeting the vibrant cafes, bars and clubs of Paris.
French president Francois Hollande vowed France will wage “merciless” war on IS after the jihadists claimed responsibility for the attacks.
Grief, alarm and resolve spread across Europe as officials raced to piece together information on the attackers.
Paris prosecutor Francois Molins said three groups of attackers, including seven suicide bombers, carried out the “act of barbarism” that shattered a Parisian Friday night.
A Syrian passport found near the body of one was linked to a man who entered the European Union through a Greek island last month.
Mr Molins said as well as the 129 people who were killed, 352 people were injured, with 99 in a critical condition.
He said the attackers in the Bataclan concert hall, where 89 people died, mentioned Syria and Iraq during their rampage.
Seven attackers launched gun attacks at Paris cafes, detonated suicide bombs near France’s national stadium and killed hostages inside the concert venue during a show by an American rock band.
Ahsan Naeem, a 39-year-old filmmaker, said he has been to many of the places that were attacked.
“I’ve seen dozens of gigs at the Bataclan. Eaten at the Petit Cambodge. Sat outside Le Carillon on so many nights,” said Mr Naeem, who has lived in Paris for seven years.
“All those places will have been full of my people. My friends. My acquaintances.”
Late on Saturday, a crowd of up to 250 people gathered for an impromptu candlelight vigil at the Place de la Republique, the site of a massive demonstration in the wake of the Charlie Hebdo killings earlier this year.
Adrien Chambel, a 27-year-old law student, said the crowd was much sparser than in January. “You feel that people are petrified,” he said.
Mr Hollande, who declared three days of national mourning and raised the nation’s security to its highest level, called the carnage “an act of war that was prepared, organised, planned from abroad with internal help”.
The president said France would increase its military efforts to crush IS.
He said France - which is part of a US-led coalition bombing suspected IS targets in Syria and Iraq and also has troops fighting Islamic militants in Africa - “will be merciless toward the barbarians of Islamic State group”.
IS claimed responsibility in an online statement in Arabic and French circulated by supporters.
It was not immediately possible to confirm the authenticity of the claim, which bore the group’s logo and resembled previous verified statements from the group.
The statement called Paris “the capital of prostitution and obscenity” and mocked France’s air attacks on suspected IS targets in Syria and Iraq, saying France’s air power was “of no use to them in the streets and rotten alleys of Paris”.
Many of Paris’s most famous tourist attractions closed down on Saturday, including the Eiffel Tower, the Louvre Museum and the Disneyland theme park east of the capital.
Some 3,000 troops were deployed to help restore order and reassure a frightened populace.
French interior minister Bernard Cazeneuve announced that all public demonstrations would be banned until Thursday and local governments throughout the country would have the option to impose nightly curfews.
Parisians expressed shock, disgust and defiance in equal measure. Some areas were quiet, but hundreds queued outside a hospital near the Bataclan concert hall to donate blood.
As a shrine of flowers expanded along the pavement, a lone guitarist sang John Lennon’s peace ballad Imagine.
The authorities said seven attackers died, six in suicide bombings, a new terror tactic in France. They said police shot the other assailant, exploding his suicide vest.
Mr Molins said all seven attackers wore identical suicide vests containing the explosive TATP.
He said one was identified from fingerprints as a French-born man with a criminal record.
The owner of the passport entered the EU in October through Leros, one of the islands that tens of thousands of people fleeing war and poverty in Syria and elsewhere have been using as a gateway.
Mr Molins said the Syria-linked attacker was not known to French intelligence services.
If the attack does involve militants who travelled to Europe amid millions of refugees from the Middle East, the implications could be profound.
Poland’s prospective minister for European affairs, Konrad Szymanski, said that in light of the attacks, Poland would not comply with an EU plan to accept refugees unless it received “guarantees of security”.
The attack brought an immediate tightening of borders as Mr Hollande declared a state of emergency and announced renewed border checks. Germany also stepped up border checks.
Belgian authorities conducted raids in Brussels and arrested three people near the border with France after a car with Belgian numberplates was seen close to the Bataclan. Mr Molins said a French national was among the three arrested.
The militants launched six gun and bomb attacks over the course of 20 minutes on Friday in areas of the capital packed with people.
Three suicide bombs targeted spots around the national Stade de France stadium, in the north of the capital, where Mr Hollande was watching a France-Germany football match. Fans inside the stadium recoiled at the sound of explosions but the game continued.
Around the same time, fusillades of bullets shook a trendy Paris neighbourhood as gunmen targeted a string of crowded cafes.
The attackers next stormed the Bataclan, which was hosting the American rock band Eagles of Death Metal. They opened fire on the panicked audience and took many hostage.
As police closed in, three detonated explosive belts, killing themselves, according to Paris police chief Michel Cadot.
Another assailant detonated a suicide bomb on Boulevard Voltaire, near the music hall, the prosecutor’s office said.
Video shot by Le Monde reporter Daniel Psenney from his balcony captured scenes of panic as people fled the Bataclan, some bloodied and limping, others dragging two bodies.
Three people could be seen clinging to upper-floor balcony railings in a desperate bid to stay out of the line of fire.
The Paris carnage was the worst in a series of attacks claimed by IS in the past three days.
On Thursday, twin suicide bombings in Beirut killed at least 43 people and wounded more than 200, and 26 people died on Friday in Baghdad in a suicide blast and a roadside bombing that targeted Shiites.
The militant group also said it bombed a Russian plane that crashed in Egypt’s Sinai Peninsula on October 31, killing 224 people.
IS also suffered significant reversals this week, with Kurdish forces launching an offensive to retake the strategic Iraqi city of Sinjar and the US military saying it had probably killed Mohammed Emwazi, the British-accented militant known as “Jihadi John” who is seen in grisly IS beheading videos.
The Pentagon also said an American airstrike targeted and probably killed Abu Nabil, a senior IS leader in Libya.
France has been on edge since January, when Islamic extremists attacked the satirical newspaper Charlie Hebdo, which had run cartoons of the Prophet Muhammad, and a kosher grocery. Twenty people died in those attacks, including three shooters.
A French judicial official says a Seat car with suspected links to the attacks has been found by police in Montreuil, a suburb about four miles east of Paris.
The official could not immediately confirm if it was the same black Seat linked to the gun attacks on the Le Carillon bar and the Le Petit Cambodge restaurant in Rue Alibert in the city’s 10th district.
Paris prosecutor Francois Molins said on Saturday that gunmen armed with automatic weapons pulled up in that model of car before opening fire, killing 15 people and injuring 10.
Serbian police say the owner of a passport found near one of the suicide bombers entered the country on October 7 from Macedonia - part of a wave of asylum-seekers crossing the Balkans toward western Europe.
Police said in a statement that the man, identified only as A.A., formally requested asylum in Serbia. The statement says it is the same passport holder registered as entering Greece on October 3.
The Syrian passport was found next to the body of a man who attacked France’s national stadium on Friday night.
Officials in Greece say the passport’s owner entered through Leros, one of the eastern Aegean islands that tens of thousands of people fleeing war and poverty have been using as a gateway into the European Union.