PARIS — The Eiffel Tower stood dark in a symbol of mourning Saturday night as France struggled to absorb the deadliest violence on its soil since World War II: coordinated gun-and-suicide bombing attacks across Paris that left at least 129 people dead and 352 injured.

President Francois Hollande vowed that France would wage “merciless” war on the Islamic State group, which claimed responsibility for the mayhem, as investigators raced to track down their accomplices and uncovered possible links to networks in Belgium and Syria.

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.

He said the attackers in the Bataclan concert hall, where 89 people died, mentioned Syria and Iraq during their rampage. Of the hundreds wounded in the six attacks, 99 were in critical condition.

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 — an attack on the heart of the pulsing City of Light.

Ahsan Naeem, a 39-year-old filmmaker, said he’s been to many of the places that were attacked Friday.

“I’ve seen dozens of gigs at the Bataclan. Eaten at the Petit Cambodge. Sat outside Le Carillon on so many nights,” said Naeem, who has lived in Paris for seven years. “All those places will have been full of my people. My friends. My acquaintances.”

Late 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,” Chambel said.

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, organized, 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 U.S.-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.”

The Islamic State group 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 top tourist attractions closed down 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.

In Washington, a crowd gathered outside the White House for a vigil near a statue of the Marquis de Lafayette, the French general for whom the park in front of the executive mansion is named.

In New York, the Empire State Building went dark for a second night in sympathy for people of Paris, while One World Trade Center was little with the colors of the French flag.

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.

The attacks, on an unusually balmy November Friday evening, struck at the heart of Parisian nightlife, including at a soccer match, which draws together spectators of all social classes and backgrounds.

Paris Mayor Anne Hidalgo said the attacks had targeted the Paris of diversity, “probably because this example of living together, which is so strong in our city, is unbearable for fanatical people.”

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 sidewalk, a lone guitarist sang John Lennon’s peace ballad, “Imagine.”

Authorities said seven attackers died, six in suicide bombings, a new terror tactic in France. Authorities said police shot the other assailant, exploding his suicide vest. Police have detained two relatives of the one attacker who has been identified so far, the prosecutor’s spokeswoman said.

Molins, the prosecutor, said all seven attackers wore identical suicide vests containing the explosive TATP.

Molins said one was identified from fingerprints as a French-born man with a criminal record.

In addition, a Syrian passport found near the body of another attacker was linked to a man who entered the European Union through a Greek island last month.

Officials in Greece said the passport’s owner entered 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 into the European Union. Molins said the Syria-linked attacker was not known to French intelligence services.

If the attack does involve militants who traveled 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 Hollande declared a state of emergency and announced renewed border checks. Germany also stepped up border checks.

Belgian authorities conducted raids in a Brussels neighborhood and arrested three people near the border with France after a car with Belgian license plates was seen close to the Bataclan theater. Molins said a French national was among the three arrested.

The militants launched six gun and bomb attacks over the course of 20 minutes 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 Hollande was watching a France-Germany soccer match. Fans inside the stadium recoiled at the sound of explosions, but the match continued.

Around the same time, fusillades of bullets shook a trendy Paris neighborhood as gunmen targeted a string of crowded cafes.

The attackers next stormed the Bataclan concert hall, 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.

A tall 38-year-old concert-goer named Sylvain collapsed in tears as he described escaping from the chaos during a lull in gunfire.

“There were shots everywhere, in waves,” Sylvain told The Associated Press. “I lay down on the floor. I saw at least two shooters, but I heard others talk. They cried, ‘It’s Hollande’s fault.’ I heard one of the shooters shout, ‘Allahu Akbar.’”

He spoke on condition that his full name not be used out of concern for his safety.

The Paris carnage was the worst in a series of attacks claimed by the Islamic State group 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 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 Oct. 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 U.S. military saying it had likely 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 likely killed Abu Nabil, a top Islamic State 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.

Paris resident Olivier Bas was among several hundred people who gathered at the site of the Bataclan massacre Saturday, laying flowers and lighting candles only a few hundred yards (meters) from where a police officer was murdered during the Charlie Hebdo attacks.

Although Paris was quiet and jittery, Bas said that he intended to go out for a drink — “to show that they won’t win.”

Meanwhile, French authorities continued their investigation. They are particularly concerned about the threat from hundreds of French Islamic radicals who are known to have traveled to Syria and have returned home, potentially with skills to mount attacks.

“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?” said Brian Michael Jenkins, a terrorism expert.

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Associated Press writers Raphael Satter, Thomas Adamson, Lori Hinnant, Angela Charlton, Sylvie Corbet, Jerome Pugmire, Philippe Sotto, Samuel Petrequin and John Leicester in Paris; Maamoun Youssef in Cairo, John-Thor Dahlburg in Brussels, Jill Lawless in London and Shawn Pogatchnik in Dublin contributed to this report.