France's president has urged the nation to stay united as it hunts gunmen who killed 12 people at the office of a satirical newspaper.

Speaking on national television about the slain journalists, Francois Hollande said: "They are today our heroes."

The attack at the offices of Charlie Hebdo weekly newspaper was France's deadliest attack in decades. The newspaper has caricatured Islamic extremists and the Prophet Muhammad.

The attackers are at large, and Mr Hollande called for them to be chased down, judged and punished.

He said France would hold a national day of mourning and fly flags at half-staff for three days.

France has seen a rise in religion-related tensions. Mr Hollande told the French "Let us unite ... and we will win."

The three gunmen shouted "Allahu akbar!" - Arabic for "God is great" - as they opened fired in the military-style attack on the newspaper located near Paris' Bastille monument.

Mr Hollande called the slayings "a terrorist attack without a doubt," and said several other attacks have been thwarted in France in recent weeks. Fears have been running high in France and elsewhere in Europe that jihadis returning from conflicts in Syria and Iraq will stage attacks at home.

France raised its security alert to the highest level and reinforced protective measures at houses of worship, shops, media offices and transportation. Schools closed across Paris, although thousands of people jammed Republique Square near the site of the shooting to honour the victims, holding aloft pens and papers reading "Je suis Charlie" - "I am Charlie."

There was no immediate claim of responsibility for the attack, which also left four people critically wounded, and was condemned by world leaders as an attack on freedom of expression, but praised by supporters of the militant Islamic State group.

Clad all in black with hoods and carrying assault rifles, the attackers forced one of the cartoonists arriving at the office building with her young daughter to open the door with a security code.

The staff was in an editorial meeting and the gunmen headed straight for the paper's editor, Stephane Charbonnier - widely known by his pen name Charb - killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman. Minutes later, two men strolled out to a black car waiting below, calmly firing on a police officer, with one gunman shooting him in the head as he writhed on the ground, according to video and a man who watched in fear from his home across the street.

The witness said the attackers were so methodical he first mistook them for France's elite anti-terrorism forces. Then they fired on the officer.

"They knew exactly what they had to do and exactly where to shoot. While one kept watch and checked that the traffic was good for them, the other one delivered the final coup de grace," he said. "They ran back to the car. The moment they got in, the car drove off almost casually."

The witness added: "I think they were extremely well-trained, and they knew exactly down to the centimetre and even to the second what they had to do."

Eight journalists, a guest and two police officers were killed, said Paris prosecutor Francois Molins, giving a partial breakdown of the 12 dead. Among those killed were Bernard Maris, an economist who was a contributor to the newspaper and was heard regularly on French radio, and cartoonists Georges Wolinski and Berbard Verlhac, better known as Tignous.

"Hey! We avenged the Prophet Muhammad! We killed Charlie Hebdo," one of the men shouted in French, according to a video shot from a nearby building and broadcast on French TV. Other video showed two gunmen in black at a crossroads who appeared to fire down one of the streets.

The video showed the killers moving deliberately and calmly. One even bent over to toss a fallen shoe back into the small black car before it sped off. The car was later found abandoned in northern Paris, the prosecutor said, and they hijacked a Renault Clio. There were conflicting accounts of whether the manhunt was for two or three attackers.

Corinne Rey, the cartoonist who said she was forced to let the gunmen in, said the men spoke fluent French and claimed to be from al Qaida. In an interview with the newspaper l'Humanite, she said the entire shooting lasted perhaps five minutes, and she hid under a desk.

The security analyst group Stratfor said the gunmen appeared to be well-trained, "from the way they handled their weapons, moved and shot. These attackers conducted a successful attack, using what they knew, instead of attempting to conduct an attack beyond their capability, failing as a result".

Both al Qaida and the Islamic State group have repeatedly threatened to attack France. Just minutes before the attack, Charlie Hebdo had tweeted a satirical cartoon of the Islamic State's leader giving New Year's wishes.

Charlie Hebdo has been repeatedly threatened for its caricatures of the Prophet Muhammad and other sketches. Its offices were firebombed in 2011 after an issue featured a caricature of the prophet on its cover. Nearly a year later, the publication again published Muhammad caricatures, drawing denunciations from the Muslim world because Islam prohibits the publication of drawings of its founder.

Another cartoon, released in this week's issue and entitled "Still No Attacks in France," had a caricature of a jihadi fighter saying "Just wait - we have until the end of January to present our New Year's wishes." Charb was the artist.

"This is the darkest day of the history of the French press," said Christophe DeLoire of Reporters Without Borders.

In the winter 2014 edition of the al Qaida magazine Inspire, a so-called chief describing where to use a new bomb said: "Of course the first priority and the main focus should be on America, then the United Kingdom, then France and so on."

In 2013, the magazine specifically threatened Charb and included an article titled "France the Imbecile Invader."

President Barack Obama offered US help in pursuing the gunmen, saying they had attacked freedom of expression. He offered prayers and support for France, which he called "America's oldest ally."

Russian President Vladimir Putin also condemned the attack as a "cynical crime," and pledged cooperation in fighting terrorism.

Mohammed Moussaoui, president of the Union of French mosques, condemned the "hateful act," and urged Muslims and Christians "to intensify their actions to give more strength to this dialogue, to make a united front against extremism."

The Organisation of Islamic Cooperation based in Jiddah, Saudi Arabia, which represents 57 Muslim-majority nations, added its condemnation, saying that violence and radicalism were the biggest enemies of Islam and went against all its fundamental principles and values.