#JeSuisCharlie (“I am Charlie”) became the most-used hashtag in Twitter history (at the time) after the January 7, 2015 terrorist attack on Charlie Hebdo satirical magazine in Paris, where gunmen killed 12 people including cartoonists who had published Muhammad caricatures.

Within 48 hours, the hashtag appeared 5+ million times, with 3.5 million people marching in Paris (largest French protest since WWII liberation) on January 11.

The slogan represented solidarity with free speech and press freedom, though debates erupted over whether defending Charlie Hebdo’s provocative cartoons constituted anti-Muslim bigotry or legitimate satire.

Sources:

• BBC coverage: https://www.bbc.com/news/world-europe-30710883
• Time Magazine analysis: https://time.com/3656635/je-suis-charlie-hashtag-paris-shooting/