PARIS — At least 100 people died in a popular Paris concert hall where attackers seized hostages Friday, an official said, one of at least six terror attacks that unfolded across the city in the deadliest violence Paris has seen since World War II.

French President Francois 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 carnage 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.

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 deadly attacks by Islamic extremists in January on satirical newspaper Charlie Hebdo and a kosher grocery that left 20 dead, 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.

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.