Messages of condolence, outrage and defiance over the Paris terrorist attack on a newspaper office spread quickly around the world with thousands of people taking to the streets to protest the killings and through use of the slogan Je Suis Charlie on social media.

Many who poured into Place de la Republique in eastern Paris near the site of today's attack waved papers, pencils and pens.

Journalists led the march but most in the crowd were not from the media world, expressing solidarity and support of freedom of speech.

Similar gatherings took place at London's Trafalgar Square, in front of the Brandenburg Gate in Berlin, in Madrid, Brussels, Nice and elsewhere.

Online, the declaration "Je Suis Charlie", or "I Am Charlie" replaced profile pictures on Facebook while Twitter users showed themselves with the slogan on signs with words of support for the 12 victims who were killed at Charlie Hebdo, a weekly newspaper that had caricatured the Prophet Mohammed.

The Je Suis Charlie slogan grew into a trending hashtag on Twitter and spread to Instagram, along with an image of a machine gun with the words "Ceci n'est pas une religion", or "This is not a religion."

One user on Instagram sent out a simple black-and-white drawing of the Eiffel Tower with the message: "Pray for Paris."

Another wrote: "Islam is a beautiful religion. This is not what we see on TV. Terrorists are not real Muslims. #IamCharlie."

Masked gunmen methodically killed 12 people, including the newspaper's editor before fleeing in a car.

The newspaper's depictions of Islam have drawn condemnation and threats before. It was firebombed in 2011 and also satirised other religions and political figures.