Ian Hislop, editor of British satirical magazine Private Eye, said the victims of the Charlie Hebdo shooting "paid a very high price for exercising their comic liberty".
At least 12 people died in the attack, believed to include editorial staff members Jean Cabu, Stephane "Charb" Charbonnier, Bernard "Tignous" Verlhac and Bernard Maris.
The offices of the magazine were previously firebombed after it published cartoons mocking the Prophet Muhammad.
In a statement, journalist and writer Hislop said: "I am appalled and shocked by this horrific attack – a murderous attack on free speech in the heart of Europe.
"I offer my condolences to the families and friends of those killed – the cartoonists, journalists and those who were trying to protect them.
"They paid a very high price for exercising their comic liberty."
He added: "Very little seems funny today."
The massacre this morning was France's deadliest terror attack in at least two decades.
The masked gunmen, who later fled, were heard shouting "Allahu Akbar" as they stormed the headquarters of Charlie Hebdo.
Charlie Hebdo's editor-in-chief, Gerard Biard, who was in London at the time of the attack, said: "I don't understand how people can attack a newspaper with heavy weapons. A newspaper is not a weapon of war."
The editor-in-chief of British satire website The Daily Mash has spoken of his shock at the Charlie Hebdo terror attack said: "You think if it is possible that these sorts of extremists would turn their guns on writers and satirists. It seems such an extreme possibility that when it happens it shocks you to the core."
Scotsman Rafferty set up The Daily Mash in 2007 with fellow journalist Paul Stokes and it has developed a cult following for its cutting edge satire and often off-the-wall approach. Its targets have included politicians of all British parties and Russian president Vladimir Putin.
Rafferty said the website had received a "vague threat" in its early days but nothing that had really troubled them.
He said people do get upset with the content – especially on religion – but it was important for satirists to tread close to the line.
He added: "We have to, it is the nature of satire really. It has to challenge authority and pre-conceived ideas. If it is not doing that it is not doing its job."
The top story on the website is currently an image of a recent Charlie Hebdo front page in tribute to the dead in Paris.
Rafferty added: "We are just discussing some ideas about how we might respond. The most important thing is to carry on doing what we do, business as usual for satirists.
"There is no question you feel intimidated, but you carry on."
People on social media have been using the #jesuischarlie hashtag to express solidarity with the magazine.