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.

A view shows policemen and rescue members at the scene after a shooting at the Paris offices of Charlie Hebdo, a satirical newspaper, January 7, 2015. [Photo/China Daily via agencies]

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.