One of the three suspects wanted in connection with the killing of 12 people at a satirical magazine in Paris on January 7 has turned himself in.
Hamyd Mourad, 18, reportedly surrendered to police early on January 8.
Police conducted raids on homes in the northeastern city of Reims as part of a nationwide manhunt for the other two suspects -- brothers Said, 32, and Cherif Kouachi, 34 years old.
Police identified the suspects and released photos of the brothers wanted for the brutal attack on the staff of the weekly Charlie Hebdo that also left 11 people injured, four seriously.
At least two men clad in black and carrying Kalashnikovs entered the magazine's offices, killing eight journalists, two police, and two others before escaping by car.
President Francois Hollande called the attack "barbaric" and said "the entire republic" had been attacked.
He said January 8 would be a day of mourning.
Cherif Kouachi was convicted of terrorism charges in 2008 and spent more than one year in prison.
Police described the suspects as armed and dangerous.
Meanwhile, tens of thousands of people are taking part in rallies around France and in other European cities, including London and Berlin.
Many are holding up pens and pencils in support of the journalists who were killed in the attack.
Shortly after the attack, President Francois Hollande said there was no doubt the attack had been a terrorist attack "of exceptional barbarity," adding that "several terrorist attacks were thwarted in recent weeks."
The president later announced January 8 would be a day of mourning, adding that flags will fly at half-mast across the country for three days.
Hollande also called for the unity of French citizens against "barbarity," saying: "Today, the entire republic has been attacked. The republic means freedom of expression, culture, creation, pluralism, democracy. This is what was targeted by the killers."
"Our best weapon is our unity -- the unity of all citizens in the face of this ordeal, he added. "Nothing can divide us, nothing should pit us against each other. Nothing should separate us."
France raised its alert status for Paris to the highest level following the attack.
The country 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.
Spain also said it was beefing up security around key infrastructure.
Prosecutor Francois Molins confirmed that the attackers shouted "Allahu akbar" (God is great) and said they wanted to "avenge the Prophet" as they stormed into Charlie Hebdo's offices.
The French Muslim Council, which represents France's Muslim community condemned the Paris shooting as an "extremely grave barbaric action" and urged Muslims to beware of extremist manipulation.
In the evening, tens of thousands of people joined rallies in Paris and other cities across France to pay tribute to the victims of the attacks.
The dead included editor in chief Stephane Charbonnier, known as Charb, and cartoonists known as Cabu, Tignous, and Wolinski -- household names in France.
One of the police officers who were killed was guarding Charbonnier, who had received numerous death threats over the years.
A cartoonist for Charlie Hebdo, Coco, said the attackers spoke perfect French and claimed they were linked to Al-Qaeda.
"I had gone to pick my daughter up from day care, when I arrived in front of the door of the magazine's building two armed men wearing balaclavas brutally threatened us," she said. "They wanted to get in, to go up. I dialled the code."
She said the attackers shot at staff for five minutes, and that she hid under a desk.
A video attributed to France 24.com appeared to show an attacker in a black balaclava shooting one person in the street in the head at close range before getting into a small car and leaving the scene.
Witness Benoit Bringer told a local TV station: "About a half an hour ago, two black-hooded men entered the building with Kalashnikovs. A few minutes later, we heard lots of shots," he said, adding that the men were then seen fleeing the building.
WATCH: Amateur video filmed in Paris on January 7 shows gunmen shooting a wounded policeman on the sidewalk outside an office where attackers killed at least 12 people. (Reuters)
UN Secretary-General Ban Ki-moon condemned the "horrendous" attack in Paris, branding it "a direct assault on a cornerstone of democracy, on the media and on freedom of expression."
The UN Security Council "strongly condemned this intolerable terrorist act targeting journalists and a newspaper."
U.S. President Barack Obama said he condemned the "horrific shooting," adding that he had directed his administration to provide any assistance needed "to help bring these terrorists to justice."
During a joint press conference with Polish Foreign Minister Grzegorz Schetyna in Washington, U.S. Secretary of State John Kerry said the freedom of expression that Charlie Hebdo represented "will never be eradicated by any act of terror."
German Chancellor Angela Merkel said in a statement that the shooting in Paris is not only an attack on French citizens, but on "freedoms of press and speech."
British Prime Minister David Cameron condemned the attack on Twitter.
"The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press," he said in a tweet.
Kremlin spokesman Dmitry Peskov said that Russian President Vladimir Putin expressed condolences and that Russia "decisively condemns terrorism in all its forms."
Iraqi Prime Minister Haidar al-Abadi said the attack highlighted the need to close ranks against militants. "This confirms that terrorism threatens all the countries of the world, and not Iraq alone," he said.
PHOTO GALLERY: Images from the attack
A Vatican spokesman, Father Ciro Benedettini, condemned the shooting as "abominable," and called it "a double act of violence...both an attack against people as well as against freedom of the press."
The Arab League and Al-Azhar, Sunni Islam's most prestigious center of learning, both condemned the attack.
Dalil Boubakeur, the leader of the French Council of the Muslim Faith, said the attack was an "immensely barbaric act also against democracy and freedom of the press" and said its perpetrators could not claim to be true Muslims.
The New York-based Committee to Protect Journalists (CPJ) described the attack as a brazen assault on free expression. "Journalists must now stand together to send the message that such murderous attempts to silence us will not stand," CPJ Deputy Director Robert Mahoney said.
Iran also condemned the killing, with Foreign Ministry spokeswoman Marzieh Afkham saying, "All acts of terrorism against innocent people are alien to the doctrine and teachings of Islam."
But Afkham also said that "making use of freedom of expression...to humiliate the monotheistic religions and their values and symbols is unacceptable."
Satirical weekly Charlie Hebdo drew international attention in February 2006 when it reprinted cartoons of the Prophet Muhammad that had originally appeared in Danish daily Jyllands-Posten, sparking a wave rage across the Muslim world.
Its former editorial offices were firebombed in November 2011 when it published a cover featuring a cartoon of Muhammad and under the title "Charia Hebdo."
Despite being taken to court under antiracism laws, the weekly continued to publish controversial cartoons of the Muslim prophet.
In September 2012, Charlie Hebdo published cartoons of a naked Muhammad as violent protests were taking place in several countries over a low-budget film, titled "Innocence of Muslims," which was made in the United States and insulted the prophet.
Its latest tweet, earlier on January 7, was a cartoon of the leader of the Islamic State militant group, Abu Bakr al-Baghdadi.
The January 7 attack came on the same day a new book imagining France in 2022 coming under Islamic rule hit French bookshops. Soumission (Submission) is the latest novel by French author Michel Houellebecq.
Charlie Hebdo's cover this week refers to Submission.
The attack also comes after tens of thousands of people again took to the streets in several German cities in rival rallies for and against a new group that opposes what it claims is the Islamization of Europe.
In its latest show of strength, Patriotic Europeans Against the Islamization of the West (PEGIDA) drew some 18,000 people to a demonstration on January 5 in Dresden and smaller crowds to rallies in other German cities.
With reporting by AP, Reuters, AFP, and CNN