It might have seemed completely unplanned to many adults, but lots of kids knew all about what was going to happen on tuesday night.
E-mail
The whole thing was sparked off by the power of e-mail!
Newsround spoke to one of the kids on the protest. She told us that she had received an e-mail telling her what was going to happen.
The message said that kids should protest outside Parliament about the war, and when they should do it.
The e-mail quickly went round hundreds of kids all round London.
She described how, in her playground at break time the next day, a bunch of kids brought out banners they had made, and walked out.
Almost the whole school followed!
Teachers couldn't stop them
Their teachers tried to stop them, but the children ran to the Underground.
The children started prostesting in Parliament Square, but were moved on by police at 1pm.
They started waving their banners and chanting anti-war slogans outside Downing Street, where police kept them off the road.
Hear Story
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