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".
"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."
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, praising the pair 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.