The US military has blocked Internet service at the Guantanamo Bay Naval Base in Cuba following online hacking threats from the group, Anonymous.
Anonymous launched Operation Guantanamo on May 18 this year to support prisoners who are on a hunger strike.
The hacker group said Guantanamo Bay is an ongoing war crime that it will no longer tolerate.
Anonymous said: "We are outraged. We, the people and Anonymous, will not allow the most expensive prison on Earth to be run without any respect for international laws."
"We stand in solidarity with the Guantanamo hunger strikers. We will shut down Guantanamo." it added.
"On May 17 to May 19, to coincide with the 100th day of the hunger strike, we urge everyone to join global actions on the ground and hacktivist protests as well as twitterstorms, email bombs, and fax bombs, in three days of nonstop action."
Earlier this year, Anonymous hacked a US government website following the death of Aaron Swartz, an American computer programmer and Internet activist.
Last year, Anonymous launched a series of cyber attacks against websites in Israel following its assault on Gaza.
