BENGHAZI, Libya — Libya's rebels have arbitrarily detained dozens of civilians suspected of supporting ruler Moammar Gadhafi and at least one has died after apparently being tortured while in custody, Human Rights Watch said 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 Sunday. It remains unclear how many are civilians because rebel authorities often do not distinguish them from fighters, seeing all Gadhafi 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-Gadhafi 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 Gadhafi.

The report also said at least 10 former security officials in the Gadhafi 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 Gadhafi's forces have conducted "wide-scale arrests" of suspected opponents. The Gadhafi regime does not provide information on its detainees.

Also Monday, NATO launched airstrikes around Tripoli in the morning and afternoon as the alliance appeared to be increasing the frequency of their strikes around the Libyan capital — the stronghold of Gadhafi's four-decade-old regime. British Maj. Gen. Nick Pope said Royal Air Force planes struck Gadhafi's military intelligence headquarters.

An old Italian colonial building that Libyan government officials said was bombed Monday appeared half gone, with piles of rubble strewn about its elegant, pastel-colored archways. Elsewhere, books still sat on a bookshelf, but a second-floor door opened to a room that was no longer there.

Government officials said the 1920s building housed parliamentary committees and charities. A strike on the same building in early May appeared to expose a large, reinforced basement.

As a NATO fighter jet thundered above, Libyan Deputy Foreign Minister Khaled Kaim told reporters on a government-organized tour that he didn't know why the building had been hit.

In Brussels, NATO Secretary-General Anders Fogh Rasmussen said Monday he will ask some of the more reluctant allies to step up their participation in the military campaign against Gadhafi. He did not mention specific nations but said he would make it a "focus" of the two-day NATO meeting opening Wednesday at alliance headquarters in Brussels.

Also Monday, two rebels were killed in fighting with Gadhafi's forces in the eastern oil town of Brega, 125 miles (200 kilometers) southwest of the de facto rebel capital of Benghazi, a medic said.

Gadhafi forces fired mortars at a graveyard in the town of Ajdabiya, a front line town in the rebel-held east.

After the strike, rebel fighters pursued government forces west to Brega, where two rebels were killed and one was injured by government shelling, the medic said, speaking on condition of anonymity because he was not authorized to talk to reporters.

It was unclear if any government soldiers were killed because Libyan authorities don't provide casualty information.

Two civilians were killed in the western Nafusa mountain town of Zintan when a rocket fired by government troops struck their house, Jumaa Ibrahim of the region's rebel military council said via Skype.

Rebel fighters took control of three nearby towns and broke a siege on a third last week.

The largest, Yifran, is 45 miles (70 kilometers) southwest of Tripoli and had been occupied by government troops for weeks when rebel fighters retook it on June 3.

Ben Hubbard in Cairo contributed reporting.