A man lights a candle during a vigil outside the Consulate General of France to pay tribute to the victims of the suspected Islamist attack on satirical magazine Charlie Hebdo in Paris, in San Francisco, California January 7, 2015.[Photo/Agencies]

The attack was condemned by world leaders.

President Barack Obama offered US help in pursuing the gunmen, saying they had attacked freedom of expression. He offered prayers and support for France, which he called "America's oldest ally."

British Prime Minister David Cameron said his country stood united with France,

"We stand squarely for free speech and democracy. These people will never be able to take us off those values," Cameron said in the House of Commons.

Russian President Vladimir Putin also condemned the attack as a "cynical crime," and pledged cooperation in fighting terrorism.

"I think all of Europe is crying today," said Italian Premier Matteo Renzi. "All the free world is crying. All men and women who believe in freedom and reason are crying."

Salman Rushdie, who spent years in hiding after his novel, "The Satanic Verses," drew a death edict from Iran's religious authorities, said all must stand with Charlie Hebdo "to defend the art of satire, which has always been a force for liberty and against tyranny, dishonesty and stupidity."

Mohammed Moussaoui, president of the Union of French mosques, condemned the "hateful act," and urged Muslims and Christians "to intensify their actions to give more strength to this dialogue, to make a united front against extremism."

The Organization of Islamic Cooperation based in Jiddah, Saudi Arabia, which represents 57 Muslim-majority nations, added its condemnation, saying that violence and radicalism were the biggest enemies of Islam and went against all its fundamental principles and values.

A tweet from an al-Qaida representative who communicated Wednesday with The Associated Press said the group was not claiming responsibility for the attack, but called it "inspiring."

Supporters of militant Islamic groups praised it. One self-described Tunisian loyalist of al-Qaida and the Islamic State group tweeted that the attack was well-deserved revenge against France.

The hashtag (hash) JeSuisCharlie was trending as people expressed support for the weekly and for journalistic freedom. The weekly's website collapsed earlier Wednesday but was later restored.

It was the deadliest attack on journalists since 2009, when 32 journalists were killed in an ambush on a political convoy in the southern Philippines.

Philippe Val, one-time Charlie Hebdo chief, raised the possibility of publishing a special edition of the newspaper, saying "a way of speaking has been exterminated."

"We must respond, because we must testify for them," he told RTL radio.