SAN JUAN, Puerto Rico — A federal judge in Washington has blocked the Pentagon from transferring a Guantanamo Bay detainee to Tunisia, where he allegedly faces torture, according to a ruling unsealed yesterday that marked a milestone in the treatment of detainees.

The order by District Judge Gladys Kessler was unprecedented as a direct intervention in the case of a prisoner at Guantanamo Bay, where some 330 men accused of links to Al Qaeda or the Taliban are held, according to a human rights group and the detainee’s lawyers.

“This is the first time since Congress tried to strip court jurisdiction over detainees that a court stepped in and said to the administration, ‘Hey wait. You can’t do what you say you want to do,'”Jennifer Daskal, the senior counterterrorism counsel for Human Rights Watch, said.

Judge Kessler said that detainee Mohammed Abdul Rahman, who has a heart condition, was convicted in absentia in Tunisia, sentenced to 20 years in prison and allegedly would face torture there, demonstrating “the devastating and irreparable harm he is likely to face if transferred.”

In her ruling on October 2 that was kept under seal until yesterday, Judge Kessler granted a preliminary injunction to halt the Defense Department’s move to transfer Mr. Rahman to the North African nation.