French President Francois Hollande said on Saturday the attacks in Paris that killed 129 people were "an act of war" organized from abroad by Islamic State with internal help.
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"Faced with war, the country must take appropriate action," he said, without elaborating.
Gunmen and bombers attacked restaurants, a concert hall and a sports stadium at locations across Paris on Friday, killing at least 129 people and wounding 200, many of them critically.
Early on Saturday afternoon, ISIS took responsibility for the carnage in an official statement.
Hollande said he would address parliament on Monday in an extraordinary meeting and the country would observe three days of official mourning for the victims of Friday's attacks.
Paris residents were urged to remain inside on Saturday. Many businesses were closed and events scheduled for the weekend, including sports matches, a U2 concert and a premiere of Steven Spielberg's new film, were cancelled.
Some Paris synagogues operated without security, as Orthodox Jews were unaware of the order not to gather and arrived as usual. Services were cancelled at the city's cathedrals and churches.
A Paris city hall official said four gunmen systematically slaughtered at least 87 young people attending a rock concert at the Bataclan music hall. Anti-terrorist commandos eventually launched an assault on the building, and the gunmen detonated explosive belts when they entered. Dozens of survivors were rescued, and bodies were still being recovered on Saturday morning.
Some 40 more people were killed in five other attacks in the Paris region, the city hall official said, including an apparent double suicide bombing outside the national stadium, where Hollande and the German foreign minister were watching a friendly soccer international.
The coordinated assault came as France, a founder member of the U.S.-led coalition waging airstrikes against Islamic State fighters in Syria and Iraq, was on high alert for terrorist attacks ahead of a global climate conference due to open later this month.
In its claim of responsibility, Islamic State said the attacks were a response to France's campaign against its fighters.
Earlier on Saturday, Islamic State redistributed a video that first appeared on the internet a year ago, threatening to attack France if bombings of its fighters continued.
"As long as you keep bombing you will not live in peace. You will even fear travelling to the market," said a bearded Arabic-speaking militant, flanked by other fighters.
The video urged Muslims who are unable to travel to Syria to wage "holy war" in France.
"Indeed you have been ordered to fight the infidel wherever you find him - what are you waiting for? There are weapons and cars available and targets ready to be hit," said an Islamic State militant, flanked by other fighters, in the video. "Even poison is available, so poison the water and food of at least one of the enemies of Allah."
The location of the Islamic State fighters in the video was not clear and it was not possible to determine when it was filmed, but the message was unmistakable.
8 assailants dead
A spokesman for Paris Public Prosecutor Francois Molins said eight assailants had died, seven of whom had blown themselves up with explosive belts at various locations, while one had been shot dead by police.
Sources close to the investigation said a Syrian passport had been found near the body of one of the suicide bombers.
The head of Paris police said all attackers are believed to be dead, but authorities are searching for possible accomplices.
"The terrorists, the murderers raked several cafe terraces with machine-gun fire before entering (the concert hall). There were many victims in terrible, atrocious conditions in several places," police prefect Michel Cadot told reporters.
First state of emergency since WWII
After being whisked from the soccer stadium near the blasts, Hollande declared a nationwide state of emergency — the first since the end of World War II — and announced the closure of France's borders to stop perpetrators escaping.
The Paris metro railway was closed and schools, universities and municipal buildings were ordered to stay shut on Saturday. However some rail and air services are expected to run.
"This is a horror," the visibly shaken president said in a midnight television address to the nation before chairing an emergency cabinet meeting.
He later went to the scene of the bloodiest attack, the Bataclan music hall, and vowed that the government would wage a "merciless" fight against terrorism.
All emergency services were mobilized, police leave was cancelled, 1,500 army reinforcements were drafted into the Paris region and hospitals recalled staff to cope with the casualties.
"It's terrible. We haven't seen anything like this in a very long time," Emmanuel Stene of the French Red Cross told Haaretz.
Radio stations broadcast warnings to Parisians to stay home and leave the streets and urged residents to give shelter to anyone caught out in the street.
'Revenge for Syria'
The deadliest attack was on the Bataclan, a popular concert venue where the Californian rock group Eagles of Death Metal was performing. The concert hall is just a few hundred meters from the former offices of the satirical weekly Charlie Hebdo, target of a deadly attack by Islamist gunmen in January.
Some witnesses in the hall said they heard the gunmen shout Islamic chants and slogans condemning France's role in Syria.
An attacker shouted "Allahu akbar" as he fired into crowd, witnesses told AFP.
France has been on high alert ever since the attacks on the Charlie Hebdo newspaper and a kosher supermarket in Paris in January killed 18 people.
Those attacks briefly united France in defense of freedom of speech, with a mass demonstration of more than a million people. But that unity has since broken down, with far-right populist Marine Le Pen gaining on both mainstream parties by blaming immigration and Islam for France's security problems.
It was not clear what political impact the latest attacks would have less than a month before regional elections in which Le Pen's National Front is set to make further advances.
The governing Socialist Party and the National Front suspended their election campaigns. Ex-president Nicolas Sarkozy, now leader of the main conservative opposition party, said, "The terrorists have declared war on France" and backed the state of emergency and border closure.
Hollande cancelled plans to travel to Turkey at the weekend for a G20 summit.
U.S. President Barack Obama and German Chancellor Angela Merkel led a global chorus of solidarity with France and UN Secretary-General Ban Ki-moon condemned the "despicable attacks" and demanded the release of the hostages.
Israeli Prime Minister Benjamin Netanyahu ordered the country's security and intelligence agencies to give France every assistance in the wake of the attack. Israel stands "shoulder to shoulder" with France, he said in a statement on Friday.
Julien Pearce, a journalist from Europe 1 radio, was inside the concert hall when the shooting began. In an eyewitness report posted on the station's website, Pearce said several very young individuals, who were not wearing masks, entered the hall during the concert, armed with Kalashnikov assault rifles and started "blindly shooting at the crowd."
"There were bodies everywhere," he said.
The gunmen shot their victims in the back, finishing some off at point-blank range before reloading their guns and firing again, Pearce said, after escaping into the street by a stage door, carrying a wounded girl on his shoulder.
Toon, a 22 year-old messenger who lives near the Bataclan, was going into the concert hall with two friends at around 10.30 P.M. (2130 GMT) when he saw three young men dressed in black and armed with machine guns. He stayed outside.
One of the gunmen began firing into the crowd. "People were falling like dominoes," he told Reuters. He said he saw people shot in the leg, shoulder and back, with several lying on the floor, apparently dead.
Two explosions were heard near the Stade de France in the northern suburb of Saint-Denis, where the France-Germany friendly soccer match was being played. The blasts were audible on television during the live broadcast.
A witness said one of the detonations blew people into the air outside a McDonald's restaurant opposite the stadium.
The match continued until the end, but panic broke out in the crowd as rumors of the attack spread, and spectators were held in the stadium and assembled spontaneously on the pitch.
Police helicopters circled the stadium as Hollande was rushed back to the interior ministry to deal with the situation.
In central Paris, shooting erupted in mid-evening outside a Cambodian restaurant in the capital's 10th district.
Eighteen people were killed when a gunman opened fire on Friday night diners sitting at outdoor terraces in the popular Charonne area nearby in the 11th district.
The prosecutor mentioned five locations in close proximity where shootings took place around the same time.
The Paris carnage came within days of attacks claimed by Islamic State militants on a Shi'ite Muslim district of southern Beirut in Lebanon, and a Russian tourist aircraft which crashed in Egypt's Sinai Peninsula.
Earlier on Friday, the United States and Britain said they had launched an attack in the Syrian town of Raqqa on a British Islamic State militant known as "Jihadi John", but it was not certain whether he had been killed.