WikiLeaks founder Julian Assange has described Bradley Manning as "the quintessential whistleblower" after the US soldier was convicted of espionage charges.
He accused President Barack Obama of "national security extremism" after an American military judge convicted Bradley Manning over the biggest leak of classified documents in US history.
Praising Manning as "the most important journalistic source that the world has ever seen", Mr Assange said he did not receive a fair trial and called for the verdict to be overturned.
"It can never be that conveying true information to the public is espionage,"Assange said from inside the Ecuadorean Embassy in London, his home for more than a year.
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Five people were killed and dozens more were injured after a car and knife attack in the capital on Wednesday afternoon.
Among those expected to attend the ceremony will be former US President Bill Clinton and ex-Northern Ireland first minister Peter Robinson.
Five people have been killed and 40 injured after a car and knife attack in London on Wednesday.