Charlie Hebdo (Charlie Weekly) is well known for courting controversy with satirical attacks on political and religious leaders.
Two assailants were captured on video calmly leaving the scene after the shooting. A police union official said the assailants remained at liberty and there were fears of further attacks.
The satirical newspaper has been firebombed in the past after publishing cartoons joking about Muslim leaders.
France Info radio said police had confirmed 10 injured. Police informed Reuters that of the 10 wounded, five were injured critically. The news channel quoted a witness as saying he saw the incident from a building nearby in the heart of the French capital.
Some of the best-known cartoonists in France were among the 12 killed when gunmen stormed the office, AFP reported a judicial source as saying.
Editor-in-chief Stephane Charbonnier, known asCharb, and the cartoonists known asCabu,Tignous and Wolinski were among those killed.
This is a terrorist attack, there is no doubt about it,” President Francois Hollande told reporters after rushing to the scene of the attack.
“An act of indescribable barbarity has just been committed today in Paris,” he said.
“Measures have been taken to find those responsible, they will be hunted for as long as it takes to catch them and bring them to justice.”
His government raised France’s security level to the highest notch and scheduled an emergency cabinet meeting.
US President Barack Obama condemned the “terrorist attack” and pledged US assistance.
“We are in touch with French officials and I have directed my administration to provide any assistance needed to help bring these terrorists to justice,” he said.
British Prime Minister David Cameron described the attack as sickening.
German Chancellor Angela Merkel also condemned the “despicable” attack in a condolence letter to President Hollande.
“I was shocked to learn of the despicable attack on the newspaper in Paris,” the German leader wrote.
“It is also an attack on freedom of expression and the press, a key component of our free democratic culture, which cannot be justified,” she added.
Turkish Foreign Minister Mevlut Cavusoglu said his country condemned all forms of “terror”, but said terrorism and increasing Islamophobia in Europe were “interconnected”.
“We must fight against increasing racism, xenophobia and Islamophobia in Europe which threaten all our values. We must also fight against any form of terrorism,” he said.
Qatar expressed “strong” condemnation of the attack in Paris.
“Such acts that target unarmed civilians contradict all principles and moral and human values,” the Qatar’s foreign ministry said in a statement.
The Arab League and Al-Azhar, Sunni Islam’s most prestigious centre of learning, both condemned the deadly attack as well.
Al-Azhar condemned the “criminal attack,” saying that “Islam denounces any violence”, in remarks carried by Egypt’s state news agency MENA.
In a separate statement to AFP, Al-Azhar senior official Abbas Shoman said the institution “does not approve of using violence even if it was in response to an offence committed against sacred Muslim sentiments”.
The Telegraph quoted a broadcast journalist with Europe1 News as saying, “Several men in black cagoules were heard to shout ‘the Prophet has been avenged’.”
Earlier on, the Guardian in it its live updates quoted its reporter as saying: “The journalist Martin Boudot, from the Premières Lignes agency, has posted this video from the roof of a building situated close to the Charlie Hebdo building. we can hear gunshots and voices who cry ‘Allahu akbar’.”