WikiLeaks founder Julian Assange called US soldier Bradley Manning a 'hero' and said he expected him to appeal after a military judge convicted him of espionage.
Assange said the verdict had set a "dangerous precedent" and was an example of "national security extremism" from the Obama administration.
He told journalists at a small press conference in London's Ecuadorian embassy that he "expects that the case will be appealed".
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"Bradley Manning's alleged disclosures have exposed war crimes, sparked revolutions and induced democratic reforms," he argued.
"He is the most important journalistic source the world has ever seen."
London-based rights group Amnesty International on Tuesday accused the US of "a serious overreach" in its unsuccessful pursuit of charges of "aiding the enemy".
"The government's priorities are upside down," it said in a statement issued Tuesday.
"The US government has refused to investigate credible allegations of torture and other crimes under international law despite overwhelming evidence.
"Yet they decided to prosecute Manning who it seems was trying to do the right thing.
"You investigate and prosecute those who destroy the credibility of the government by engaging in acts such as torture which are prohibited under the US Constitution and in international law.
"It's hard not to draw the conclusion that Manning's trial was about sending a message: the US government will come after you, no holds barred, if you're thinking of revealing evidence of its unlawful behaviour," it added.
Assange, who has been holed up in the embassy since June last year in an attempt to avoid extradition to Sweden to face sex claims, said Manning was "unquestionably heroic".
He accused Obama of betrayal over his treatment of "whistleblowers" and insisted Manning's crimes had no victim except "the US government's wounded pride".
The administration's "attacks on Bradley Manning are not a sign of strength but a sign of weakness", he added.
He drew comparisons with former CIA operative Edward Snowden, who is currently wanted in the US over another security leak, calling the pair heroes for "willing to risk their liberty and possibly their lives" to bring information to the public.
In all, Manning was found guilty of 20 of 22 counts related to his leaking of a huge trove of secret US diplomatic cables, government records and military logs to the WikiLeaks website.
