Many held up placards saying “Je suis Charlie” (I am Charlie), referring to a hashtag that is trending on Twitter in solidarity with the victims.
Piles of pens – symbolizing freedom of expression – and candles have been laid across the square.
Tens of thousands of people have also joined rallies in other cities across France.
A major manhunt has been launched in Paris for three gunmen who shot dead 12 people at the office of the satirical magazine Charlie Hebdo.
Eight journalists, including the magazine’s editor, and two policemen were among the dead.
Protests over the killings are being held in cities across France. It is the country’s deadliest attack in decades.
President Francois Hollande called it a “cowardly murder” and declared a day of national mourning on Thursday, January 8.
Charlie Hebdo‘s website, which went offline during the attack, is displaying the single image of “Je suis Charlie” on a black banner. Other major newspapers are displaying similar banners.
The latest tweet on Charlie Hebdo‘s account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.
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