The ACLU in a statement called on the U.S. Congress to investigate what it called "the gross abuse of power committed under his (Rumsfeld's) watch.
"Donald Rumsfeld's resignation is a step in the right direction," said Anthony D. Romero, ACLU executive director.
"Rumsfeld is responsible for the torture and abuse of detainees in U.S. military custody and must be held accountable for the failures that occurred on his watch," Romero said.
The defense secretary, who announced his resignation Wednesday, "has placed the blame on junior military members and has been nothing but derelict in his duty," Romero said. "Congress must initiate an immediate and exhaustive investigation into hi s six-year-long record of unlawful activity, violations of the rule of law and complicity in the executive branch abuse of power."
The ACLU said that, as a result of a U.S. Freedom of Information Act lawsuit it had undertaken, "more than 100,000 pages of government documents have been released detailing the torture and abuse of detainees in Iraq, Afghanistan and Guantánamo Bay."
The ACLU said that a hearing in that case, Ali v. Rumsfeld, was due to be heard on Dec. 8. "In June 2006, the Supreme Court in Hamdan v. Rumsfeld ruled that the Bush administration policy on detention, orchestrated by Rumsfeld, was illegal," the ACLU statement said.
The ACLU said it had also "filed a lawsuit to uncover details of Pentagon surveillance of peace groups and law-abiding Americans who have attended anti-war protests."
"The documents obtained by the ACLU reveal that the Pentagon shared information on activists with other government agencies through the Threat and Local Observation Notice (TALON) database," the organization said.