TRIPOLI, Libya — As rebel leaders pleaded with their fighters to avoid taking revenge against “brother Libyans,” many rebels were turning their wrath against migrants from sub-Saharan Africa, imprisoning hundreds for the crime of fighting as “mercenaries” for Col. Muammar el-Qaddafi without any evidence except the color of their skin.

Many witnesses have said that when Colonel Qaddafi first lost control of Tripoli in the earliest days of the revolt, experienced units of dark-skinned fighters apparently from other African countries arrived in the city to help subdue it again. Since Western journalists began arriving in the city a few days later, however, they have found no evidence of such foreign mercenaries. Still, in a country with a long history of racist violence, it has become an article of faith among supporters of the Libyan rebels that African mercenaries pervaded the loyalists’ ranks. And since Colonel Qaddafi’s fall from power, the hunting down of people suspected of being mercenaries has become a major preoccupation.

Human rights advocates say the rebels’ scapegoating of blacks here follows a similar campaign that ultimately included lynchings after rebels took control of the eastern city of Benghazi more than six months ago. The recent roundup of Africans, though, comes at a delicate moment when the new provisional government is trying to establish its credibility. Its treatment of the detainees is emerging as a pivotal test of both the provisional government’s commitment to the rule of law and its ability to control its thousands of loosely organized fighters. And it is also hoping to entice back the thousands of foreign workers needed to help Libya rebuild.

Many Tripoli residents — including some local rebel leaders — now often use the Arabic word for “mercenaries” or “foreign fighters” as a catchall term to refer to any member of the city’s large underclass of African migrant workers. Makeshift rebel jails around the city have been holding African migrants segregated in fetid, sweltering pens for as long as two weeks on charges that their captors often acknowledge to be little more than suspicion. The migrants far outnumber Libyan prisoners, in part because rebels say they have allowed many Libyan Qaddafi supporters to return to their homes if they are willing to surrender their weapons.

The detentions reflect “a deep-seated racism and anti-African sentiment in Libyan society,” said Peter Bouckaert, a researcher with Human Rights Watch who visited several jails. “It is very clear to us that most of those detained were not soldiers and have never held a gun in their life.” In a dimly lighted concrete hangar housing about 300 glassy-eyed, dark-skinned captives in one neighborhood, several said they were as young as 16.

In a reopened police station nearby, rebels were holding Mohamed Amidu Suleiman, a 62-year-old migrant from Niger, on allegations of witchcraft. To back up the charges, they produced a long loop of beads they said they had found in his possession. He was held in a segregated cell with about 20 other prisoners, all African migrants but one. “We have no water in the bathroom!” one prisoner shouted to a guard. “Neither do we!” the guard replied. Most of the city has been without running water to bathe, flush toilets or wash clothes since a breakdown in the water delivery system around the time that Colonel Qaddafi fled. But the stench, and fear, of the migrants was so acute that guards handed visitors hospital masks before they entered their cell. Outside the migrants’ cage, a similar number of Libyan prisoners occupy a less crowded network of rooms.

Osama el-Zawi, 40, a former customs officer in charge of the jail, said his officers had allowed most of the Libyan Qaddafi supporters from the area to go home. “We all know each other,” he said. “They don’t pose any kind of threat to us now. They are ashamed to go out in the streets.” But the “foreign fighters,” he said, were more dangerous. “Most of them deny they were doing it,” he said, “but we found some of them with weapons.” A guard chimed in: “If we release the mercenaries, the people in the street will hurt them.” In the crowded prison hangar, in the Tajura neighborhood, the rebel commander Abdou Shafi Hassan, 34, said they were holding only a few dozen Libyans — local informers and prisoners of war — but kept hundreds of Africans, including innocent Ghanaian laborers in the segregated pen. On a recent evening, the Libyan captives could be seen rolling up mats after evening prayers in an outdoor courtyard just a short distance from where the Africans lay on the concrete floor in the dark.