In a statement posted on the WikiLeaks website, he said: "When communicating with the press is 'aiding the enemy' it is the 'general knowledge among the people' itself which has become criminal.
"Just as Bradley Manning is condemned, so too is that spirit of liberty in which America was founded.
"In the end it is not Bradley Manning who is on trial. His trial ended long ago. The defendant now, and for the next 12 weeks, is the United States.
"A runaway military, whose misdeeds have been laid bare, and a secretive government at war with the public. They sit in the docks. We are called to serve as jurists. We must not turn away."
More top news
Tuesday: After a bright start, rain will gradually push northeastwards across England and Wales with strong winds developing.
Britain should have a second vote on the terms of exiting the EU if it can secure a new deal to control its borders, Jeremy Hunt has said.
Complaints have been lodged with the chairman of the 1922 committee over the neutrality of government whips.