Militias, made up mostly of Arab fighters, have exploited the power vacuum to rampage through cities, loot households and kill an unknown number of civilians, according to aid workers, doctors and local activists.
In response, some civilians have begun arming themselves, and non-Arab groups have also retaliated against militias at a small scale.
But while Khartoum had been a peaceful city before April, Darfur has been torn by decades of violence.
More than 300,000 people were killed in Darfur in the 2000s when Sudan’s former dictator, Omar Hassan al-Bashir, ordered militias, widely known as the Janjaweed, to crush a rebellion among non-Arab groups.
Its eastern region, semiarid and isolated, already has more than 400,000 refugees from Darfur living in 13 camps, which are now filling with new arrivals helped by the U.N. refugee agency.