online campaign Charlie Hebdo "I am Charlie"  #JeSuisCharlie
Thousands of people tweeted their support to press freedom in the wake of the terror attack on French satirical magazine Charlie Hebdo. Twitter

Netiznes have launched a Twitter campaign in support of press freedom, empathising with French satirical magazine Charlie Hebdo, whose Paris offices were attacked by gunmen.

Tens of thousands of people used the hashtag #JeSuisCharlie to voice their condemnation of the violence that left 12 people, including two police officers and several cartoonists, dead.

Many, including the German foreign office official Twitter account, also posted a picture of the slogan on a black background.

Paris mayor Anne Hidalgo used the photo as her profile picture.

Others tweeted past cartoons published by the magazine or the photos of the cartoonists killed.

The hashtag was started by user @joachimroncin.

French police confirmed the black-clad gunmen shouted "The Prophet has been avenged", likely referring to cartoons mocking the Prophet Mohammed and radical Islam published by the magazine.

President François Hollande described the incident as a terrorist attack.

French police have launched a manhunt for the three gunmen who fled the scene aboard a vehicle.