A large manhunt is under way in France for the three gunmen who allegedly carried out Wednesday's mass murder at the Paris headquarters of the satirical weekly paper "Charlie Hebdo."
Police identified the three as Said Kouachi, 34; his brother, Cherif, 32; and Hamyd Mourad, 18.
One suspect left his identity card in the getaway car after the killings.
The Kouachi brothers are from Paris, while Mourad is from the northeastern city of Reims. One of the brothers had been previously tried on terror charges.
Police said the shootings left at least 12 people dead. Two of the victims were police officers.
French President Francois Hollande denounced the shootings as a terrorist attack and said France's best defense against such "barbarity" was unity. That unity was later displayed when thousands staged a peaceful protest in Paris in defense of free expression.
Hollande declared Thursday would be a national day of mourning.
Late Wednesday morning, black-hooded gunmen carrying Kalashnikov rifles and a pump-action shotgun stormed the weekly's offices, bursting into an editorial staff meeting and opening fire. Terrorism experts said the gunmen appeared to have carried out their attack methodically.
Among the dead on the "Charlie Hebdo" staff were three cartoonists, including co-founder Jean “Cabu” Cabut and editor-in-chief Stephane “Charb” Charbonnier.
The gunmen said "Allahu akbar" and that they were avenging the Prophet Muhammed, Paris prosecutor Francois Molins told reporters.
After dozens of shots rang out, two assailants were seen calmly leaving the scene in a video. One police officer was seen being shot as he lay wounded.
At least 11 others were injured in the attack, including at least four critically. Police union official Rocco Contento described the scene inside the offices as “carnage.”
There was no immediate claim of responsibility for the killings, but jihadists online had repeatedly warned that "Charlie Hebdo" — well-known for courting controversy with satirical attacks on political and religious leaders and publishing numerous cartoons ridiculing the Prophet Muhammed — would pay for its ridicule.
A police union official said there were fears of further attacks. The government declared the highest state of alert, increasing security at transport hubs, religious sites, media offices and department stores as the search for the assailants got under way.
Hollande addressed the nation on television late Wednesday, saying that "an act of indescribable barbarity has just been committed today in Paris,” but insisting that "no act of barbarity will ever extinguish freedom of the press."
He said several other terror plots had been foiled in recent weeks. "We were being threatened because we are a country of freedom," he said. "And because we are a country of freedom, we will fight against these threats and we will punish the aggressors."
He said, “Today, [the slain journalists] are our heroes. And that is why tomorrow will be a day of national mourning, a decreed day. At 12 o'clock there will be a moment of contemplation in all public services."
France last year reinforced its anti-terrorism laws and was already on alert after calls from Islamist militants to attack its citizens and interests in reprisal for French military strikes on Islamist strongholds in the Middle East and Africa.
Hassen Chalghoumi, imam of the Drancy mosque in the north Paris suburb of Seine-Saint-Denis, said of the attack, "I am extremely angry. These are criminals, barbarians. They have sold their soul to hell. This is not freedom. This is not Islam, and I hope the French will come out united at the end of this."
Chalghoumi said of the gunmen: “We must be firm with them, because they want terror, they want racism, they want to pit people against each other.”
Wednesday's public protest occurred later at the Place de la Republique. Agathe Bayeurte was there with her mother, Catherine.
Bayeurte said France was a country where people could say what they think, and that she was there to defend that right.
Retired history professor Francois Mareschal arrived at the rally clasping an old copy of "Charlie Hebdo." The satirical weekly may have been targeted for mocking Islam, but Mareschal was holding an old edition that mocked Christianity.
Mareschal said he joined many in Paris in denouncing fundamentalism in all its forms. The assailants, he said, had killed some of the best cartoonists in France.
U.S., world reaction
The White House strongly condemned the attack.
"France and the great city of Paris where this outrageous attack took place offer the world a timeless example that will endure well beyond the hateful vision of these killers," President Barack Obama said, adding that America's thoughts and prayers were with the victims of the terrorist attack.
The White House said Obama had directed the administration to provide any assistance needed "to help bring these terrorists to justice."
Watch related video report by VOA's Sharon Behn
In a joint appearance with Polish Foreign Minister Grzegorz Schetyna, Secretary of State John Kerry condemned the Paris attack, saying, "Each and every American stands with you today."
Kerry said the U.S. stood with France in "solidarity and commitment" in "confronting extremism."
He also addressed the people of France in French.
Schetyna said his country was "deeply touched" by the attack, saying, "We stand together with France today."
German Chancellor Angela Merkel also condemned the shooting.
“This abominable act is not only an attack on the lives of French citizens and their security. It is also an attack on freedom of speech and the press, core elements of our free democratic culture," Merkel said.
European Union foreign ministers will discuss the fight against terrorism at their next meeting.
"The fight against terrorism, in all its forms, is regularly at the heart of the work of [EU] foreign ministers' meetings. These tragic events reinforce my determination in this sense, and I will put this point on the agenda for the next foreign ministers' meeting on January 19," EU foreign policy chief Federica Mogherini said in a statement.
U.N. Secretary General Ban Ki-moon said such violence should be a unifying force.
"This horrific attack is meant to divide; we must not fall into that trap. This is a moment for solidarity around the world. We must stand strong for freedom of expression and tolerance and stand against forces of division and hate," he said.
Watch related video report by VOA's Jeff Custer:
The New York-based Committee to Protect Journalists described the attack as a brazen assault on free expression.
The scale of the violence is appalling,” said CPJ Deputy Director Robert Mahoney. “Journalists must now stand together to send the message that such murderous attempts to silence us will not stand.”
The U.N. Security Council condemned the "barbaric and cowardly terrorist attack" at the Paris weekly's offices and called for the perpetrators to be brought to justice.
"The members of the Security Council strongly condemned this intolerable terrorist act targeting journalists and a newspaper," the 15-member council said in a statement.
U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein joined a global chorus of condemnation against the "hideous crime," in Paris. But while calling for the arrest and punishment of those responsible, Zeid cautioned against reacting with "discrimination and prejudice" against a wider group.
Upon entering the building, two gunmen headed straight for the paper's editor, Charbonnier, killing him and his police bodyguard first, Christophe Crepin, a police union spokesman, told The Associated Press.
A journalist from a press agency located just across the street described the scene to French radio, saying many shots were fired from Kalashnikov rifles. He then saw two armed and masked people leaving the building.
After fleeing into the street, the gunmen exchanged fire with officers in the first police car to respond to emergency calls and with two policemen on mountain bikes before firing repeatedly at a second patrol car.
The eyewitness said he heard yelling and shots being fired on the street, and that police and emergency services arrived a few minutes later.
Amateur video filmed from a nearby rooftop showed a gunman walking up to a wounded policeman, who had staggered from the car to a nearby sidewalk, and shooting him dead point-blank.
The gunmen drove off toward northeastern Paris. They collided with a motorist before abandoning their car on a busy square and hijacking another vehicle as they escaped into the northern suburbs.
Wednesday's attack is considered one of the worst terrorist attacks on French soil in decades.
Late last year, a man shouting “Allahu Akbar” injured 13 people by ramming a vehicle into a crowd in the eastern city of Dijon. In 2012, seven people were killed in attacks that targeted French soldiers and Jewish civilians. In 1995, a series of attacks were carried out, including a commuter train bombing that killed eight and injured 150.
'Charlie Hebdo' supporters
Thousands of people took to Facebook and Twitter in support of the publication, saying freedom of expression must be defended.
"Charlie Hebdo's" offices were firebombed in 2011 after it published an issue poking fun at the Muslim prophet.
Muslims also denounced the weekly for later publishing a series of crude comics of the prophet.
Wednesday's shooting coincided with the launch of a controversial French book depicting France's election of its first Muslim president. "Charlie Hebdo" has featured the cover of the book, "Submission," on its cover.
Meanwhile, Danish media group JP/Politikens Hus, whose newspaper Jyllands-Posten published cartoons in 2005 depicting the Prophet Muhammed, increased its security level because of the Paris shooting, an internal email showed.
Jyllands-Posten's publishing of the cartoons sparked a wave of protests across the Muslim world in which at least 50 died.
Victoria Macchi contributed to this report from Washington. Some material for this report came from Reuters and AFP.