Malala Yousafzai will be remembered by historians as starting a "liberation movement" for boys and girls all over the world, Gordon Brown has said.
The former Prime Minister said Malala and Indian activist Kailash Satyarthi were "the greatest children's champions in the world" as well as "two of my great friends".
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".