During a demonstration, a lot of people gather together to say that they are for or against something. With a demonstration, they want to influence others. Someone who demonstrates is called a demonstrator.
In a democracy, you can organize a demonstration but you have to tell the government that this demonstration is going to happen. Another rule is that the demonstrators are not allowed to bring weapons.
If rules are not abided by, then the police can stop the demonstration. This happens mainly if a demonstration turns violent or demonstrators start destroying things.
A well-known demonstration happened in Germany in 1983, where around 500,000 people demonstrated against new missiles. In 1989, about half a million people from the GDR demonstrated for freedom of speech in Berlin. In Austria, in 1993 about 300,000 people demonstrated to say that immigrants should be treated equally.