Hundreds of thousands of people took to social media to express anger and disgust at the actions of those who attacked the office of Charlie Hebdo, and their sympathy for the victims.

Black badges with the slogan 'Je Suis Charlie' (I am Charlie) in the magazine's font, have been posted by people wanting to demonstrate solidarity with the magazine.

On Twitter, British prime minister David Cameron wrote: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press." Labour front-bencher Yvette Cooper wrote: "Terrorist attack on #CharlieHebdo is barbaric. We stand w people of Paris to defend free speech & values of democracy & common humanity."

Others reflected on the part played by religion in the attack, with comedian David Walliams writing: "Executing unarmed cartoonists can never be the will of any God."

Gay rights campaigner Peter Tatchell warned of the dangers of reaction, saying: "In response to the #CharlieHebdo massacre we must not suppress liberties in the name of defending them - that's what the terrorists want".

A cartoon by Dave Pope from the Canberra Times showing a terrorist standing over a dead body saying "He drew first" was widely shared, as was a Daily Telegraph picture of an edition of Charlie Hebdo – with the offending front page cartoon pixellated out.

Muslim voices were raised against the attack. Islamic Freedom tweeted: "We strongly condemn #CharlieHebdo attack 'Because ISLAM denounces any violence'."

The Muslim Times wrote: "We condemn every act of terrorism. Human life is sacred and killing one innocent person is like triggering genocide."

Tariq Ramadan wrote: "Contrary to what was apparently said by the killers in the bombing of Charlie Hebdo's headquarteres, it is not the Prophet who was avenged, it is our religion, our values and Islamic principles that have been betrayed and tainted ... my condemnation is absolute and my anger is profound (healthy and a thousand times justified) against this horror!!!

Anne Hidalgo, the mayor of Paris, released a statement on Facebook condemning the attack and calling for a march on Thursday through Paris's Republic Square at 6pm.

She says: "I feel a sense of absolute horror at the attack... We must respond to this act through the sacred union around the principles of the Republic." Thousands are expected to attend.