A federal court has ordered the military to release 21 photographs that show mistreatment of prisoners, opening a window into what civil rights advocates call a widespread system of abuse across Iraq and Afghanistan.

The photographs provide the clearest evidence yet that detainee abuse was not limited to the infamous Abu Ghraib prison in Iraq, according to the American Civil Liberties Union, which brought the case. The decision may end a long-standing legal battle in which the government argued that releasing the photos would compromise the war effort.

The photos were taken at six different locations across Iraq and Afghanistan, the 2nd Circuit Court of Appeals wrote, and involve a greater number of detainees and military personnel than similar photographs from Abu Ghraib. The ACLU attorney who argued the case, Amrit Singh, said the material shows that prisoner abuse is a systemic problem in the military.

“It’s a resounding victory for the public’s right to hold the government accountable,” Ms. Singh said. “The Bush administration has consistently maintained that what happened at Abu Ghraib was the work of a few rogue soldiers. Well, that’s not true, because it was happening elsewhere.”

The Abu Ghraib images, which depict soldiers forcing prisoners to pose naked in sexually explicit ways, prompted a huge international backlash when they surfaced in 2004. The photographs subject to yesterday’s decision show abuse but do not involve stripping or posing, according the court.

The Department of Defense disputed the ACLU’s contention that prisoner abuse is widespread. The military has conducted 12 major reviews, assessments or investigations related to detention, a spokeswoman said, and none them found that a “policy of abuse” has ever existed.

“Department of Defense policy is and always has been to treat detainees humanely,” the spokeswoman, Cynthia Smith, said in a statement. “There have been well documented instances where that policy was not followed, and service members have been held accountable for their actions in those cases.”

The appeals judges affirmed a district court decision in 2006 that the military is required to release the materials under the Freedom of Information Act, which protects the public’s right to view government information. The military had argued that it was exempt from the law because the photos could incite violence against American forces.

Exemptions in the act do not provide “an all-purpose damper on global controversy,” the court wrote, adding that “there is a significant public interest in the disclosure of these photographs.”

Most of the images will be redacted to protect the privacy of the people captured in them. The government is in possession of other photographs that should be covered by the court’s decision, Ms. Singh said.

The images may not be released immediately because the military can appeal to the Supreme Court. The Department of Defense did not comment on the case.