In Paris, tens of thousands of people have been protesting about the importance of free speech. So why is this happening?

France is holding a day of national mourning. Today at midday the whole country held a minute's silence.

It's to pay tribute to those who lost their lives in a horrific attack on a the office of a magazine in Paris, on Wednesday. Twelve people were shot and killed.

The magazine, called Charlie Hebdo, was famous for joking about serious news stories. And it often used cartoons to poke fun at all religions, including Islam.

But some extremist Muslims said those cartoons were offensive, and threatened the magazine unless it stopped printing them.

The magazine said it wouldn't stop the cartoons. It says it has the right to print what it wants: this is known as Freedom of Speech.

Many people feel free speech is one of the most important things in the world: to be able to discuss anything - even the trickiest of political issues - without fear of being attacked, even if other people find your views insulting.

After the attack, many people all over the world showed their support on social media, for the magazine's right to free speech.

Lots of people shared cartoons of their own, with the message "I am Charlie" - in memory of the cartoonists who were killed.

World leaders spoke out too about the need to defend freedom of speech.

The head of the United Nations Ban Ki-moon called the attack "a direct assault on... democracy, on the media and on freedom of expression".

Britain's Prime Minister David Cameron said: "When it comes to a challenge to our values, the best way to stand up for our values is to repeat our belief in them - in the freedom of speech, in the rule of law, in democracy, in tolerance, in faith in the way we do things and nothing and no-one will ever be able to push us off that path".