WikiLeaks founder Julian Assange called US soldier Bradley Manning's espionage conviction an episode of "national security extremism" on Tuesday, while other supporters expressed relief that he was acquitted of the most serious charge - aiding the enemy.
Among Manning's critics, members of a Congress intelligence committee said justice was served.
The charge of aiding the enemy would have carried a potential life sentence, but Manning was convicted on other counts that, together, could also mean a life behind bars. Manning faces up to 136 years in prison if given maximum penalties.
His sentencing hearing starts Wednesday.
Manning has acknowledged giving WikiLeaks more than 700,000 battlefield reports and diplomatic cables. He has said he leaked the material to expose US military "bloodlust" and diplomatic deceitfulness but did not believe his actions would harm the country.
Assange, whose website helped Manning expose US secrets to the world, called the verdict "a dangerous precedent and an example of national security extremism."
"This has never been a fair trial," he told reporters at the Ecuadorean Embassy in London, which is sheltering him.
Glenn Greenwald, the journalist who first reported former US intelligence contractor Edward Snowden's leaks of National Security Agency surveillance programs, said Manning's acquittal on the charge of aiding the enemy represented a "tiny sliver of justice."
Anatoly Kucherena, the Russian lawyer who's been working with Snowden, said: "All cases are individual. We shouldn't take the Manning case and compare it to Snowden."
Christian Stroebele, a German lawmaker for the opposition Green Party, tweeted: "Manning has won respect by uncovering the US's murderous warfare in Iraq."
In Washington, the Republican and Democratic leaders of the House Intelligence Committee joined in a statement declaring "justice has been served today."
But the advocacy group Reporters Without Borders said the verdict was a chilling warning to whistleblowers, "against whom the Obama administration has been waging an unprecedented offensive." It said the verdict threatens the future of investigative journalism because intimidated sources might fall quiet.
Another advocate of less government secrecy, Steven Aftergood of the Federation of American Scientists, questioned whether the implications will be so dire, given the extraordinary nature of the Manning case.
"This was a massive hemorrhage of government records, and it's not too surprising that it elicited a strong reaction from the government," Aftergood said.
Outside the courtroom, Manning supporters applauded his lawyer, David Coombs, and shouted "thank you."
"Today is a good day," Coombs said, "but Bradley is by no means out of the fire."
Many supporters wore black T-shirts with "truth" on them to show they consider him a whistleblower trying to expose government misconduct.
"The government's priorities are upside down," Widney Brown, senior director of international law and policy for Amnesty International, said at the scene.
Officials have "refused to investigate credible allegations of torture and other crimes under international law despite overwhelming evidence," Brown said, but they "decided to prosecute Manning, who it seems was trying to do the right thing - reveal credible evidence of unlawful behavior by the government."
"It seems clear that the government was seeking to intimidate anyone who might consider revealing valuable information in the future," said Ben Wizner of the ACLU Speech, Privacy & Technology Project.