Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris on January 7, 2015. (Xinhua/AFP Photo)
Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris on January 7, 2015. (Xinhua/AFP Photo)

PARIS, Jan. 7 (Xinhua) -- At least 12 people were killed when two masked and armed men on Wednesday stormed into the headquarters of the satirical magazine Charlie Hebdo, known for mocking radical Islamists, in Paris.

They killed 12 people and wounded several others before leaving the scene calmly. In a video shot by an amateur and broadcasted by local media, one of the perpetrators shouted "God is the greatest" and "We have killed Charlie Hebdo. We have avenged the Prophet Mohammad."

Rushed to the shooting scene shortly after the attack, French President Francois Hollande denounced "an act of indescribable barbarity".

"Without doubt, it's a terrorist attack against an office that has been threatened several times, which is why it was protected," he added, stressing determination to arrest the shooters and crack down on terrorists while urging to preserve the country's unity.

Hollande also revealed that several terrorist attacks were stopped in recent weeks in France where anti-terrorism alert vigipirat plan has been raised to the highest level, in greater Paris area, which means a definite threat.

France, home of 5 million Muslims, the largest community in Europe, has been the target of terror threats in reprisal to its operation against Daech group in Iraq and Syria.

Hundreds of radicalized nationals and residents joined Syrian insurgents and could pose menace to the country's security when they returned home.

Hassen Chalghoumi, imam of the Drancy mosque in Paris's Seine-Saint-Denis northern suburb, said "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."

Thousands of people flocked to Republic Square in the capital to condemn the attack and say "I'm Charlie". Many other rallies were reported across the country in solidarity with the attacked.

To French Prime Minister Manuel Valls, "France is heart-stricken, every French citizen is horrified."

In a brief speech, former president Nicolas Sarkozy and head of the country's main opposition UMP, condemned this "despicable act which shocks the conscience of humanity."

"It's a a direct and savage attack at one of the most beloved republican principles, freedom of the press. We must continue to say what we want and live as we want," Sarkozy said.

Hours after the shooting, the perpetrators' whereabouts are still unknown. A massive manhunt operation is underway to hunt them down.

In a press conference, Paris Porsecutor Francois Molins said at least two men opened fire on the weekly staff and killed 10 journalists, a policeman and an economist invited by the newspaper.

Molins also added that 11 others were injured with four in critical condition, while he refused to give further details on the investigation.

In November 2011, headquarters of Charlie Hebdo was fire-bombing after it put an image of the Prophet Mohammad on its cover.

In its last published cartoons, the weekly mocked Abu Bakr al-Baghdadi, the leader of the Islamic State, which seized major towns in Iraq and Syria.

Speaking in French, U.S. Secretary of State John Kerry, who has close ties to France, condemned the attack saying "all Americans stand alongside France."

In Berlin, German Chancellor Angela Merkel also condemned the "abominable act" which hit "freedom of speech and the press, core elements of our free democratic culture."

UN Secretary General Ban Ki-moon described the attack as "a horrendous, unjustifiable and cold-blooded crime."

According to the Interior Ministry, 500 additional policemen are deployed in the capital and suburbs to secure public places, including main tourism sites, train stations and stores.

An emergency government meeting was held in Elysee. A second council is scheduled for Thursday morning.

Calm in Paris was shattered in 1995 when a terrorist bomb attack in the Paris subway killed eight people and wounded some 200 others. Charlie Hebdo shooting, which caused at least 12 deaths, is the most deadly attack in France in 50 years