Masked gunmen attacked the Paris offices of satirical weekly newspaper Charlie Hebdo on Wednesday, killing 12 people before fleeing.

French security forces launched a major manhunt in the capital after the gunmen fled the scene of the attack, The Guardian reported. Police are searching for two brothers from the Paris region and another man from the northern French city of Reims in connection with the attack, a police source told Reuters.

The attackers stormed Charlie Hebdo's Paris newsroom during an editorial meeting and began firing indiscriminately, police and prosecutors said. Witnesses told police that the gunmen shouted "we have avenged the prophet," according to Agence France-Presse . Charlie Hebdo cartoonist Corinne Rey said the gunmen spoke to her in fluent French and claimed to represent al Qaeda. The gunmen called out some of the victims' names, she told Reuters.

A video, apparently filmed by an onlooker outside the office, shows two gunmen dressed in black, firing automatic weapons down the street and shouting "Allahu Akbar." Police official Luc Poignant told the Associated Press the gunmen left in a waiting car and later switched to a stolen vehicle.

France's Interior Minister Bernard Cazeneuve confirmed the deaths of 12 people in the attack. Four others were seriously wounded, the French prosecutor said. A police official told the Associated Press that one police officer was killed, along with Charlie Hebdo's publisher, Stphane Charbonnier, and Charbonnier's police bodyguard.

French media outlets also named three Charlie Hebdo cartoonists as among the dead: Cabu, Georges Wolinski and Bernard Verlhac, who is also known as Tignous. Economist and journalist Bernard Maris, who was visiting the office, was also killed, Reuters reported . Authorities have not officially confirmed these deaths.

Prosecutor Franois Molins said during a press conference that at least two gunmen were involved in the attack. France's interior minister had said earlier that there had been three attackers.

French President Francois Hollande called the killings "a terrorist attack without a doubt," and convened an emergency meeting with top government officials. The French leader vowed that the attack will not silence freedom of the press in the country.

The National Syndicate of Journalists called for a rally in Paris in response to the attack. Le Monde reported that at least 15,000 had gathered in the capital to condemn the killings, and Parisians were seen holding up pens as a show of support for the slain journalists. A number of other gatherings were also planned in cities across the country.

Charlie Hebdo has been the target of violence in the past over its work. In 2011, its offices were firebombed after it carried a caricature of Prophet Muhammad on its cover. When the newspaper published more provocative cartoons of Prophet Muhammad a year later, it sparked violent protests across the Muslim world.

Minutes before Wednesday's attack, Charlie Hebdo tweeted a satirical cartoon of Abu Bakr al-Baghdadi, the leader of the Islamic State group.

Armed gunmen face police officers near the offices of the French satirical newspaper Charlie Hebdo in Paris on January 7, 2015. (ANNE GELBARD/AFP/Getty Images)

Media gather at the scene on the attack on Charlie Hebdos offices in Paris on January 7, 2015. (Marc Piasecki/Getty Images)

An injured person is transported to an ambulance after a shooting at the French satirical newspaper Charlie Hebdo's office in Paris, Jan. 7, 2015. (AP Photo/Thibault Camus)

Reactions poured in from around the world.

U.S. President Barack Obama condemned the attack in a statement on Wednesday and said the American government would "provide any assistance needed to help bring these terrorists to justice." U.S. Secretary of State John Kerry called the attack "horrific" and told the people of France that "each and every American stands with you today."

German Chancellor Angela Merkel called Wednesday's assault "an attack on freedom of speech and the press." She added, "This abominable act is not only an attack on the lives of French citizens and their security."

British Prime Minister David Cameron called the attack "sickening." Russian President Vladimir Putin offered his condolences to the victims' loved ones and all Parisians.

French President Francois Hollande, center, flanked with security forces gestures as he arrives outside Charlie Hebdo's office in Paris, Jan. 7, 2015. (AP Photo/Remy De La Mauviniere)

France raised its security alert to the highest level in the wake of the attack. Large stores, public transportation and the offices of newspapers are among the sites receiving additional security.

After #charliehebdo attack, metal shutters down, 4 police onsite at Le Monde newspaper via @Matgoa : #CharlieHebdo pic.twitter.com/3RuGfTKblW

Jennifer O'Mahony (@jaomahony) January 7, 2015

French soldiers patrol at the Eiffel Tower after a shooting at a French satirical newspaper in Paris, France, Jan. 7, 2015. (AP Photo/Christophe Ena)

Eline Gordts contributed to this report.This is a developing story and has been updated.