BENGHAZI, Libya—Libya’s rebels have arbitrarily detained dozens of civilians suspected of supporting ruler Muammar Qaddafi and at least one has died after apparently being tortured while in custody, Human Rights Watch said on Monday.

Since the uprising started in mid-February, rebels have seized control of much of the country’s east and scrambled to set up an administration in their de facto capital of Benghazi. Rebels also hold the western city of Misrata and smaller towns in the western mountains. Both sides have taken prisoners in the fighting.

On Monday New York-based Human Rights Watch called on Libya’s rebels to give detainees legal protection and investigate abuses, said researcher Sidney Kwiram.

“Detainees are entitled to their full due process rights, including access to a lawyer,” Kwiram said. “The concern is that if this is not addressed early, bad habits can become entrenched.”

As of May 28, rebel authorities held about 330 people, Human Rights Watch said in a report released on Sunday. It remains unclear how many are civilians because rebel authorities often do not distinguish them from fighters, seeing all Qaddafi supporters as enemies of the “revolution.”

In Benghazi at least one-third of 118 detainees were civilians, the report said. Of the 20 Human Rights Watch civilians interviewed, none said they had been abused, but none had been able to meet with a lawyer or challenge their detention in court.

The report, based on interviews with detainees and rebel officials in three rebel-controlled cities, also said bands of volunteers were rounding up people suspected of pro-Qaddafi activities.

“They are making arrests with no formal legal authority, and that creates the space for vigilante justice,” Kwiram said.

The report said one man held by such a group, Muhammad el-Dabr, died in late April, apparently after being tortured. El-Dabr, a Jordanian citizen, was suspected of spreading propaganda for Qaddafi.

The report also said at least 10 former security officials in the Qaddafi regime have been killed in the past three months.

Rebel officials were not immediately available for comment. The report quoted Mustafa Abdul-Jalil, chairman of the rebels’ National Transitional Council, as promising all detainees “a fair trial and someone to defend them.”

The report also said Qaddafi’s forces have conducted “wide-scale arrests” of suspected opponents. The Qaddafi regime does not provide information on its detainees.