WikiLeaks founder Julian Assange branded Bradley Manning's espionage conviction an episode of 'national security extremism' while other supporters expressed relief that he was acquitted of the most serious charge.
Among Manning's critics, House intelligence officials said justice was served.
In Washington, the Republican and Democratic leaders of the House Intelligence Committee joined in a statement declaring "justice has been served".
"Manning harmed our national security, violated the public's trust, and now stands convicted of multiple serious crimes," said Mike Rogers, chairman of the House Intelligence committee, and Dutch Ruppersberger, the panel's top Democrat.
Mr Assange, whose website served as the conduit for exposing Manning's spilled US secrets to the world, saw nothing to cheer in the mixed verdict.
"It is a dangerous precedent and an example of national security extremism," he told reporters at the Ecuador Embassy in London, which is sheltering him. "This has never been a fair trial."
Glenn Greenwald, the journalist, commentator and former civil rights lawyer who first reported 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".
And Christian Stroebele, a German MP for the opposition Green Party, tweeted: "Manning has won respect by uncovering the US's murderous warfare in Iraq."
But the advocacy group Reporters Without Borders said the verdict is a warning to whistle-blowers, "against whom the Obama administration has been waging an unprecedented offensive", and 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 haemorrhage of government records, and it's not too surprising that it elicited a strong reaction from the government," Mr Aftergood said.
"Does that mean that every leak from every journalist is likely to do the same?" he asked. "No it doesn't. Most journalists are not in the business of publishing classified documents, they're in the business of reporting the news, which is not the same thing. This is not good news for journalism, but it's not the end of the world, either."
Daniel Ellsberg, whose sensational leak of the Pentagon papers in the early 1970s exposed US government lies about the Vietnam War, said Manning's acquittal on aiding the enemy limits the chilling consequences of the WikiLeaks case on media freedoms.
"American democracy just dodged a bullet, a possibly fatal bullet," Mr Ellsberg said. "I'm talking about the free press that I think is the life's blood of the democracy."
Outside the courtroom, Manning supporters gave his lawyer, David Coombs, a round of applause and shouted "thank you." But they also pressed him on what the verdict meant for the soldier's fate.
"Today is a good day," Mr Coombs said, "but Bradley is by no means out of the fire."
Manning acknowledged giving WikiLeaks more than 700,000 battlefield reports and diplomatic cables, and video of a 2007 US helicopter attack that killed civilians in Iraq, including a Reuters news photographer and his driver. He said during a pretrial hearing he leaked the material to expose US military "bloodlust" and diplomatic deceitfulness but did not believe his actions would harm the country.
His defence portrayed him as a naive but well-intentioned figure. Prosecutors branded him an anarchist and traitor.
Howard "Buck" McKeon, chairman of the House Armed Services Committee, welcomed the outcome.
"Bradley Manning endangered the security of the United States and the lives of his own comrades in uniform when he intentionally disclosed vast amounts of classified data," he said. "His conviction should stand as an example to those who are tempted to violate a sacred public trust in pursuit of notoriety, fame, or their own political agenda."
Many supporters in and outside the courtroom wore black T-shirts with "truth" on them to show they consider him a whistle-blower just trying to expose government misconduct.
"The government's priorities are upside down," said Widney Brown, senior director of international law and policy for Amnesty International.
Officials have "refused to investigate credible allegations of torture and other crimes under international law despite overwhelming evidence," Ms Brown said, but "decided to prosecute Manning, who it seems was trying to do the right thing - reveal credible evidence of unlawful behaviour 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.
Overseas, Anatoly Kucherena, the Russian lawyer who has been working with Snowden, said: "All cases are individual. We shouldn't take the Manning case and compare it to Snowden."
