Amnesty International is demanding the United States drop all charges against Julian Assange, days before the WikiLeaks founder battles extradition from the UK.
The human rights organisation says the espionage charges against Assange are a chilling blow to publishers and journalists.
His extradition trial starts on February 24.
Assange is facing a US prison sentence of up to 175 years over his organisation's release of classified American diplomatic cables and Pentagon files on the Iraq and Afghanistan wars, some of which revealed US war crimes.
He has been charged with 17 counts of spying and as well as conspiring to commit computer intrusion.
"The US government's unrelenting pursuit of Julian Assange for having published disclosed documents that included possible war crimes committed by the US military is nothing short of a full-scale assault on the right to freedom of expression," Amnesty spokesman Massimo Moratti said on Friday.
"The potential chilling effect on journalists and others who expose official wrongdoing by publishing information disclosed to them by credible sources could have a profound impact on the public's right to know what their government is up to."
Amnesty also fears the Australian would face human rights violations in the US, including prolonged solitary confinement.
Mr Moratti says if the US doesn't drop the charges, this risk of rights violation should prompt the UK to refuse to extradite him.
Australian government MP George Christensen and independent MP Andrew Wilkie visited the 48-year-old for an hour-and-a-half at Belmarsh prison near London on Monday.
They believed facts expected to come to light during the trial would prompt the Australian government to intervene and were concerned about Assange's mental state.
Australian Associated Press