BEIJING - World leaders have condemned the armed attack of the headquarters of a Paris-based weekly, which killed at least 12 people.

French President Francois Hollande on Wednesday branded the shooting spree against Charlie Hebdo as a terrorist attack.

"Without doubt, it's a terrorist attack against an office that has been threatened several times, which is why it was protected," said Hollande.

Vigipirate, France's national security alert system, has been raised to Scarlet, its highest level which means a definite threat, in the greater Paris area, announced the prime minister's office on Wednesday.

Press offices, department stores, public transportation and places of worship in France will be placed under "reinforced protection," said the office.

US President Barack Obama denounced the deadly attack and pledged assistance in bringing the perpetrators to justice.

"I strongly condemn the horrific shooting at the offices of Charlie Hebdo magazine in Paris that has reportedly killed 12 people," the president said in a written statement. He described France as the "oldest ally" of the United States that has stood "shoulder to shoulder" with Washington in the fight against terrorism.

British Prime Minister David Cameron also condemned the attack. "I believe the House will join me in condemning the barbaric attack this morning on an office of a magazine in Paris," he told Westminster.

Reaffirming solidarity with the French people, Cameron added: "We stand united with the French people in our opposition to all forms of terrorism and stand squarely for free speech and democracy."

German Chancellor Angela Merkel extended condolences to Hollande, saying she was shocked to learn of the despicable attack.

"I would like to express to you and your compatriots in the hours of suffering the sympathy of the German people as well as my own sorrow, and convey my condolences to the victims' loved ones," Merkel said.

Jean-Claude Juncker, the president of the European Commission, called the attack "brutal and inhuman."

"This is an intolerable act, a barbarism that challenges us all as human and European beings," Juncker said in a statement.

NATO Secretary-General Jens Stoltenberg slammed the shooting as "a barbaric act and an outrageous attack on press freedom."

Noting that NATO stands in full solidarity with France, he said all NATO allies stand together in the fight against terrorism.

UN Secretary-General Ban Ki-moon voiced "outrage at the despicable attack," which he described as a "horrendous, unjustifiable and cold-blooded crime."

"This horrific attack is meant to divide," Ban told reporters. "We must not fall into that trap."

In Geneva, the United Nations Human Rights chief condemned the deadly shooting.

"I utterly condemn the appalling and ruthless attack on media workers and police officers in Paris earlier today," Zeid Ra'ad Al Hussein, the UN High Commissioner for Human Rights, said in a statement.

"I offer my heartfelt condolences to the people of France, especially the families, friends and colleagues of those who were shot in cold blood in their office, and of those lying critically injured in hospital," he said.

In Brussels, President of the European Council Donald Tusk said the European Union stood side by side with France.

"I am shocked by the appalling attack that took place this morning in Paris against Charlie Hebdo. It is a brutal attack against our fundamental values, against freedom of expression which is a pillar of our democracy," he said in a statement.

Tusk extended "deepest condolences" to the families and relatives of the victims and express solidarity with the French authorities and the French people.

French authorities confirmed that at least 12 people were killed in the attack on the satirical weekly and four others were seriously wounded.

In November 2011, the headquarters of Charlie Hebdo was fire-bombed after it put an image of the Islamic Prophet Muhammad on its cover.