The prosecutor’s office confirmed that “at this stage” 10 people had been killed without detailing how many had been injured.
Another source close to the investigation said the number of dead had reached 11.
Deputy Mayor of Paris Bruno Julliard earlier said “six people are seriously injured”, including a policeman. It was not clear whether these now figured among the dead.
French President Francois Hollande arrived at the scene of the shooting after rushing there and calling an emergency cabinet meeting, the presidency said.
The government raised its alert level to the highest possible in the greater Paris region.
A source close to the investigation said two men “armed with a Kalashnikov and a rocket-launcher” stormed the building in central Paris and “fire was exchanged with security forces.”
The source said a gunman had hijacked a car and knocked over a pedestrian while attempting to speed away.
The publication’s cartoonist Renaud Luzier earlier told AFP there were “casualties” after the incident.
The satirical newspaper gained notoriety in February 2006 when it reprinted cartoons of the Prophet Mohammed (P.B.U.H) that had originally appeared in Danish daily Jyllands-Posten, causing fury across the Muslim world.
Its offices were fire-bombed in November 2011 when it published a cartoon of Mohammed and under the title “Charia Hebdo”.
Despite being taken to court under anti-racism laws, the weekly continued to publish controversial cartoons of the Muslim Prophet (P.B.U.H).
In September 2012 Charlie Hebdo published cartoons of a naked Mohammed 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.
French schools, consulates and cultural centres in 20 Muslim countries were briefly closed along with embassies for fear of retaliatory attacks at the time.
Editor Stephane Charbonnier has received death threats and lives under police protection. – AFP