French President Francois Hollande said at least 127 were killed on Friday in a series of six coordinated attacks in Paris that he described as an "act of war" -- blaming ISIS for the violence that included shootings, explosions and a hostage situation at a concert hall.
"And when faced with war, the country must take appropriate decisions," Hollande said on Saturday. "An act committed by a terrorist army, DAESH [ISIS], against what we are, a free country that speaks together with the planet."
Three days of national mourning in France has been declared, according to the French president.
Eight attackers are dead, and seven died detonating suicide vests, according to Agence France-Presse, citing police. A Syrian passport was found on the body of an attacker near one of the attack sites, the Stade de France stadium, police told ABC News.
French police said they believed all of the attackers involved in the shootings and bombings were dead, but authorities were searching for accomplices.
Three hundred people were hospitalized, according to Paris medical authorities, including 80 people in life-threatening condition and 177 in somewhat critical condition. Forty three witnesses were also admitted, medical authorities said, adding that 53 people have since been discharged.
Americans were among those injured, according to the State Department.
"The U.S. government is working closely with French authorities to identify American victims," said Deputy Spokesperson Mark Toner. "We are aware there are Americans among the injured, and are offering them the full range of consular assistance."
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According to Agence France-Presse, citing a police source, three of the attackers at the concert hall died when they detonated their suicide belts as police closed in. A fourth was shot dead by police. AFP said that three more attackers were dead outside the Stade de France soccer stadium and one on a street in eastern Paris.
In the wake of the attacks, Hollande said he was imposing border checks and declaring a state of emergency. Anyone deemed dangerous could be placed under house arrest. Hollande called the attacks "unprecedented" and President Obama called them an attack "on all of humanity."
The two leaders shared a phone call on Friday evening where Obama expressed his condolences. "The President reiterated the United States' steadfast, unwavering support for the people of France, our oldest ally and friend, and reaffirmed the offer of any necessary support to the French investigation," the White House said.
In a tweet, the Elysee Palace said: "terrorists capable of such atrocities should know that they will face a determined and united France."
The attacks took place at the theater, the Stade de France and four other locations in the 10th and 11th districts.
At Stade de France, Germany was playing France in soccer. Hollande was at the game at the time and was evacuated.
One witness, Margot Schmorak, told ABC News via Skype that one of the attacks "was around the corner from us, about a half a block down."
Schmorak and three colleagues "were sitting outside on sidewalk ... we heard a bunch of gun shots. Maybe 8 to 10, and then there was kind of a pause, and there was a few more."
"We actually said to each other, 'Is that gunshots or is it firecrackers or something else?' And then one of my friends actually said, 'I think it's gunshots.' And we saw people running from around the corner," she said.
Another witness, Emilioi Macchio, was at a bar when he heard gunshots, according to the Associated Press.
"It sounded like fireworks," Macchio said, the AP reported.
Police in Paris were recommending that residents avoid going out unless absolutely necessary and the subway system was shut down. All public buildings in Paris, including schools, museums, libraries and town halls, were closed indefinitely starting Saturday. The U.S. Embassy in Paris advises official U.S. personnel and citizens to shelter in place.
The Department of Homeland Security said it is closely monitoring the events in Paris and said there was no specific or credible threat to the United States. DHS said it's in contact with its counterparts in the area and "will not hesitate to adjust our security posture, as appropriate, to protect the American people."
Eagles of Death Metal, an American rock band that was performing at Bataclan on Friday, said in a statement, "We are still currently trying to determine the safety and whereabouts of all our band and crew. Our thoughts are with all of the people involved in this tragic situation."
Gérard Araud, the French ambassador to the U.S., said he was devastated by the carnage.
President Obama said from the White House Friday that, while the details of the attacks in France are unknown, the U.S. stands together with France "in the fight against terrorism and extremism."
Obama called France an "extraordinary counter-terrorism partner."
"Paris itself represents the timeless values of human progress," Obama said, adding that the "American people draw strength from the French people's commitment to life, liberty and the pursuit of happiness."
A federal law enforcement source said the FBI set up a “command post” to help monitor and respond to the attacks. The FBI is actively trying to help French authorities, but as of now, U.S. authorities do not know who is responsible.
The Associated Press contributed to this report.