A man holds a sign reading "I am Charlie" in Marseille, France, on January 7, 2015, following an attack by unknown gunmen on the offices of the satirical weekly Charlie Hebdo in Paris

The hashtag #JeSuisCharlie (I am Charlie), in tribute to the 12 people killed in the attack on the Paris offices of satirical newspaper Charlie Hebdo, has now been used in more than 3.5 million tweets, Twitter said Thursday.

The trending slogan reached a peak at 2030 GMT on Wednesday, hours after the bloody assault by two gunmen apparently acting from Islamist motives. Then, some 6,500 were using the tag.

On Instagram, nearly 650,000 photos were shared with #JeSuisCharlie on them at 1600 GMT on Wednesday. By 0100 GMT Thursday, the trend had declined to 52,000 shares.

An Instagram spokesman told AFP that Wednesday's peak coincided with a Paris demonstration in homage to the victims killed that gathered at least 35,000 people.

Thursday was more about sharing cartoons, either from Charlie Hebdo or from other cartoonists around the world reacting to the massacre, as well as an image declaring "Je Suis Charlie" with a black background.