As France vowed a "merciless" response to an unprecedented terrorist atrocity, police arrested a man in Brussels after searches linked him to the six co-ordinated attacks.

The French paper Liberation reported a police operation was underway in the Molenbeek neighbourhood of Brussels, according to reports on Belgian television. It is believed the operation is related to the Paris attacks, and so far one man has been arrested.

French media reported unnamed Belgian police sources saying three of the terrorists were from Molenbeek, and one of the vehicles used in the attacks had Belgian number plates.

The swift police action comes as the death toll rose on Saturday to 127. More than 180 remain in hospital.

The French newspaper Liberation reported that one of the gunman who carried out the Bataclan attack has been confirmed as a French-born extremist who was known to police. He was identified by his fingerprints.

It also reported that a Syrian passport which was found on the body of the attacker at the Stade de France had passed through Greece as a refugee in October.

"The holder of the passport passed through the island of Leros on Oct. 3, 2015, where he was identified according to EU rules," said Greece's deputy minister in charge of police, Nikos Toskas, in a statement.

A Greek police source said the passport's owner was a young man who had arrived in Leros with a group of 69 refugees and had his fingerprints taken by authorities there. Police declined to give his name.

Toskas did not know if the passport was checked by other countries through which the holder possibly passed on his way to France. The police source said French authorities had asked other countries in Europe, including Greece, to check on the passport.

According to French media reports, all the attackers are believed to have died, most by detonating explosive suicide belts they were wearing.

Also on Saturday, a France-bound airliner was grounded for checks at Amsterdam's Schiphol airport following a threatening tweet that was received concerning the flight, Dutch border police said.

A police spokesman said that authorities had been searching the flight, operated by Franco-Dutch airline Air France-KLM and which had been due to depart around 1.45pm, for around an hour.

Dutch Prime Minister Mark Rutte announced heightened security measures for all traffic going from or to France.

Security services have told Parisians to stay indoors this weekend amid concerns that other conspirators or supporters may remain at large.

Islamic State on Saturday claimed responsibility as President Francois Hollande declared the attacks an "act of war".

In an official statement the group said its fighters carried out the attacks in various locations in the heart of the capital which were carefully studied.

On Friday night around 9pm, two suicide bombers detonated their weapons at cafes close to the Stade de France, where Hollande was watching the national football team play Germany.

He was evacuated by helicopter, and the terrified, confused crowd spilled onto the field.

Meanwhile, gunmen wielding AK-47 assault rifles opened fire at three cafes in the north-east of Paris, where diners were sitting at tables alongside the streets. Scores of people were shot, and more than 30 died.

A group of attackers also entered the nearby Bataclan concert hall, where a rock concert was in progress.

In the dark, they calmly strafed the young audience, who panicked, fled or fell to the ground.

Radio reporter Julien Pearce was inside the theatre, where the American rock band Eagles of Death Metal was performing.

He told CNN he saw two terrorists with Kalashnikov rifles, wearing black clothes, enter the room and start firing randomly into the crowd.

People screamed and fell to the floor, he said.

"It lasted for 10 minutes, 10 horrific minutes … we heard so many gunshots," Pearce said.

The "calm, determined" attackers reloaded three or four times, saying nothing as they fired.

"They said nothing. They just shot. They were just shooting … it was a bloodbath."

Another eyewitness said the attackers fired from a balcony down into the crowd. A third person claimed to have heard the attackers shouting about France's role in the Syria conflict.

Benjamin Cazenoves wrote on Facebook: "I'm still at the Bataclan​. First floor. Hurt bad! There are survivors inside. They are cutting down everyone. One by one. First floor quickly!!!!

He added: "Alive. Just cuts... Carnage... Dead bodies everywhere."

According to early reports three of the attackers blew themselves up, and a fourth died as police stormed the concert hall.

The head of emergency at Georges-Pompidou Hospital said victims sustained injuries similar to that of war victims, their bodies peppered with bullets.

In some of the worst cases, there were many bullet wounds over the victim's entire body due to the use of heavy weapons and firearms by the terrorists.

The attacks come just 10 months after the attacks on the satirical magazine Charlie Hebdo and a kosher supermarket by Islamist extremists.

The Bataclan​ theatre is just a few blocks from Charlie Hebdo's former office where the January attack took place.

Mr Hollande visited the theatre hours after the attack, which he called "an abomination and a barbaric act".

"France will stand firm," he said. "We are going to fight and our fight will be merciless."

US President Barack Obama led a chorus of world leaders condemning the attacks.

Mr Obama said US will do "whatever it takes to bring these terrorists to justice". France was an "extraordinary terrorism partner" and an attack on the French people was an attack on "all humanity".

British Prime Minister David Cameron expressed solidarity and offered any help needed.

Australian Prime Minister Malcolm Turnbull, who was in Berlin on his way to the G20 Leaders Summit in Turkey, described the attacks as an assault on all humanity and "the work of the devil".

UN Chief Ban Ki-moon condemned the attacks as "despicable", while NATO Secretary-General Jens Stoltenberg said the Atlantic defence alliance would stand with France "strong and united" against terrorism.

France has declared a state of emergency, for only the second time since World War II.

All festivals and public gatherings have been cancelled on Saturday. Schools and colleges are shut and the public transport system is also expected not to run.

Parisians have been told to stay home.

The country's borders were closed as a temporary measure, though incoming flights and train travel operations were still running at the time of reporting.

More than 1500 extra soldiers have been deployed to the streets of Paris and to key transport hubs.

Police and security services have begun a manhunt for those who may have aided or known of the attack.

The government's security cabinet is expected to meet again on Saturday morning. It is the deadliest violence in Paris since World War II.

At the end of the month, Paris is due to host a key United Nations meeting on climate change, to be attended by scores of world leaders.

However many countries are now urgently reviewing whether the capital is safe for such a high-level meeting.

with Reuters