"This is madness. It is the end of national security journalism and the first amendment", WikiLeaks said via Twitter on Thursday.
Earlier in the day, the Justice Department announced that it indicted Assange on 17 additional charges, including conspiracy to obtain, receive and disclose national defense information as well as conspiring to commit computer intrusion.
Assange's lawyer said in a separate statement that the US charges are "unprecedented" and that the criminal prosecution against Assange represents a threat to all journalists.
If convicted, Assange faces a maximum penalty of ten years in prison on each count except for conspiracy to commit computer intrusion, for which he faces a maximum penalty of five years in prison.
Assange was arrested in London on 11 April, after Ecuador revoked his asylum status. He was then sentenced to 50 weeks in jail for jumping bail back in 2012, when he took refuge at the Ecuadorian embassy in London to avoid extradition to Sweden over a sexual assault investigation, which had since been dropped. The United States is seeking Assange's extradition on charges of conspiracy to commit computer intrusion.
Assange gained fame after WikiLeaks published a large number of classified documents, including some that exposed abuses of power and war crimes committed by US forces in Afghanistan and Iraq, and at the Guantanamo Bay detention camp.