Clooney was speaking in London at a conference for media freedom attended by delegations from more than 100 countries.
British Foreign Secretary Jeremy Hunt and Canadian Foreign Minister Chrystia Freeland joined Clooney on stage at the event which was hosted by Britain with the Canadian government.
The conference aimed to defend media freedom from restrictive practices by governments, encourage participants to develop plans to legislate for a free press, help improve journalists' safety and counter disinformation.
Clooney also criticised U.S. President Donald Trump for ''vilifying'' members of the media.
Trump regularly mounts attacks on the news media and political opponents on Twitter.
He says he can bypass what he labels unfair media coverage by speaking directly using social media.