PARIS (AP) — A series of unprecedented attacks on popular night spots killed more than 100 people in the deadliest violence to strike France since World War II, officials said. President Francois Hollande condemned it as a terrorist attack and pledged that France would stand firm against its foes.

The worst carnage was at a concert hall hosting an American rock band, where scores of people were held hostage and attackers hurled explosives at their captives. Police who stormed the building, killing two attackers, encountered a bloody scene of horror inside.

Hollande declared a state of emergency and announced that he was closing the country's borders. The violence spread fear through the city and exceeded the horrors of the Charlie Hebdo attack just 10 months ago.

Paris police officials said security officials had launched an assault on the concert hall, killing at least two attackers. One described "carnage" inside the building, saying the attackers tossed explosives at the hostages.

In addition to the deaths at the concert hall, a police official said 11 people were killed in a Paris restaurant in the 10th arrondissement and other officials said at least three people died when bombs went off outside a stadium.

All of the officials spoke on condition of anonymity because they were not authorized to be publicly named in the quickly moving investigation.

There was no immediate claim of responsibility for the attacks, and no clear picture of how many attackers were involved and if any were on the run. Jihadists on Twitter immediately praised the attack and criticized France's military operations against Islamic State extremists.

Hollande, who had to be evacuated from the stadium when the bombs went off outside, said in a televised address that the nation would stand firm and united.

"This is a terrible ordeal that again assails us," he said. "We know where it comes from, who these criminals are, who these terrorists are."

U.S. President Barack Obama, speaking to reporters in Washington, called the attacks on Paris "outrageous attempt to terrorize innocent civilians" and vowed to do whatever it takes to help bring the perpetrators to justice. He called the attacks a "heartbreaking situation" and an "attack on all of humanity."

Earlier Friday, two explosions were heard outside the Stade de France stadium north of Paris during a France-Germany friendly football match. A police union official said there were two suicide attacks and a bombing that killed at least three people.

The official, Gregory Goupil of the Alliance Police Nationale, whose region includes the area of the stadium, said explosions went off simultaneously near two entrances and a McDonalds.

An Associated Press reporter in the stadium Friday night heard two explosions loud enough to penetrate the sounds of cheering fans. Sirens were immediately heard, and a helicopter was circling overhead.

The attack comes as France has heightened security measures ahead of a major global climate conference that starts in two weeks, out of fear of violent protests and potential terrorist attacks. Hollande canceled a planned trip to this weekend's G-20 summit in Turkey, which was to focus in large part on growing fears of terrorism carried out by Islamic extremists.

Emilio Macchio, from Ravenna, Italy, was at the Carillon restaurant that was targeted, having a beer on the sidewalk, when the shooting started. He said he didn't see any gunmen or victims, but hid behind a corner, then ran away.

"It sounded like fireworks," he said.

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, including the three attackers. The Charlie Hebdo attackers claimed links to extremists in Yemen, while the kosher market attacker claimed ties to the Islamic State group.

This time, they targeted young people enjoying a rock concert and ordinary city residents enjoying a Friday night out.

One of at least two restaurants targeted Friday, Le Carillon, is in the same general neighborhood as the Charlie Hebdo offices, as is the Bataclan, among the best-known venues in eastern Paris, near the trendy Oberkampf area known for a vibrant nightlife. The California-based band Eagles of Death Metal was scheduled to play there Friday night.

The country has seen several smaller-scale attacks or attempts since, including an incident on a high-speed train in August in which American travelers thwarted an attempted attack by a heavily armed man.

France's military is bombing Islamic State targets in Syria and Iraq and fighting extremists in Africa, and extremist groups have frequently threatened France in the past.

French authorities are particularly concerned about the threat from hundreds of French Islamic radicals who have travelled to Syria and returned home with skills to stage violence.

Though who was responsible for Friday night's violence remained a mystery, the Islamic State is "clearly the name at the top of everyone's list," Brian Michael Jenkins, a terrorism expert and senior adviser to the president of RAND Corp., said.

Jenkins said the tactic used — "multiple attackers in coordinated attacks at multiple locations" — echoed recommendations published in extremist group's online magazine, Dabbiq, over the summer.

"The big question on everyone's mind is, were these attackers, if they turn out to be connected to one of the groups in Syria, were they homegrown terrorists or were they returning fighters from having served" with the Islamic State group, Jenkins said. "That will be a huge question."

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Associated Press writers Angela Charlton, Jerome Pugmire, Samuel Petrequin, Jamey Keaten and John-Thor Dahlburg contributed to this story.