The top three trending subjects on Twitter in the UK at the time Campaign published this story were #CharlieHebdo, #JeSuisCharlie and then #ParisShooting. The hashtag #CharlieHebdo had been mentioned more than 1.8 million times on Twitter at the time of writing.
The hastags are being used to show solidarity with the 12 people who were killed by hooded gunmen in Paris today after an attack on the satirical magazine, Charlie Hebdo, and also support the freedom of expression.
Earlier today, Francois Hollande, the French president, tweeted: "La liberté sera toujours plus forte que la barbarie. Notre meilleure arme, c'est notre unité." which translates as "freedom will always be stronger than barbarism. Our best weapon is our unity".
It has been reported that the attack was in response to Charlie Hebdo publishing cartoons that criticised Islam.
(the Je Suis Charlie image, which was posted on the website of Charlie Hebdo earlier today)
Prominent cartoonists including the Dutch political commentator Ruben Oppenheimer took to Twitter to publish their works in response to the attack and the Independent posted a piece by its cartoonist, Dave Brown.
(a cartoon by Oppenheimer in reponse to the attacks)
(A drawing by the Independent's cartoonist, Dave Brown).