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US President Barack Obama has led global condemnation of the shooting at a Paris weekly which has left 12 people dead.
Governments lined up to offer their support after masked men armed with Kalashnikov automatic rifles opened fire at the offices of French satirical newspaper Charlie Hebdo, in what President Francois Hollande said was a "terrorist attack".
The Committee to Protect Journalists said it was a "brazen assault on free expression in the heart of Europe", while Reporters Without Borders called it a "black day".
"Our thoughts and prayers are with the victims of this terrorist attack and the people of France at this difficult time," Obama said on Wednesday.
He offered US government help in tracking down the culprits, and added: "Time and again, the French people have stood up for the universal values that generations of our people have defended.
British Prime Minister David Cameron condemned the shooting as "sickening" and "barbaric", while German Chancellor Angela Merkel called it "despicable"; sentiments reflected across European capitals.
Russian President Vladimir Putin "resolutely condemns terrorism in all its forms", a spokesman said.
'Unspeakable atrocity': Abbott
In a post on Twitter on Thursday, Mr Abbott labelled the attack an "atrocity".
"If you don't like something, you don't read it; you don't kill people you disagree with," the tweet said.
UN Secretary General Ban Ki-moon condemned the "horrendous" attack, branding it an assault on the media and freedom of expression.
"It was a horrendous, unjustifiable and cold-blooded crime. It was also a direct assault on a cornerstone of democracy, on the media and on freedom of expression," Ban told reporters at UN headquarters on Wednesday.
"This horrific attack is meant to divide. We must not fall into that trap," he added.
There was also condemnation from Muslim states of the gunmen, who were heard to shout "we have avenged the prophet" and "Allahu Akbar" ("God is greatest"), according to French police.
The massacre took place after years of confrontation between Charlie Hebdo and Islamists infuriated by what they saw as the publication's attacks on their religion.
Al-Azhar, Sunni Islam's most prestigious centre of learning, called the attack "criminal" and said "Islam denounces any violence", according to Egyptian news agency MENA, while the Arab League also condemned the attack.
Charlie Hebdo's editor-in-chief Stephane Charbonnier, known as Charb, and the cartoonists known as Cabu, Tignous and Wolinski were among those killed.
"The scale of the violence is appalling," said Robert Mahoney, deputy director of the New York-based Committee to Protect Journalists.
"Journalists must now stand together to send the message that such murderous attempts to silence us will not stand."
Security was reportedly stepped up Wednesday at the Danish newspaper that itself provoked angry and sometimes deadly protests worldwide by publishing a series of cartoons of the Prophet Mohammed in 2005.
Charlie Hebdo had reprinted the cartoons in 2006.
