JUBA, South Sudan — The security forces went house to house, rounding up civilians by the dozens and binding the wrists of some with wire, survivors said. Some were summarily shot in the street, they said, while others were hauled off to crowded cells. Bodies of the executed were tossed into shallow graves, one recalled. Another jail where civilians had been taken reeked of death, a witness said.

“We thought that the war was fought between the soldiers,” said Peter Nhial, 30, one of many in a crowd of desperate people to describe attacks on civilians.

Little more than a week after political tensions between South Sudan’s leaders erupted into clashes in the streets of the capital, the crisis has broadened into a societal conflict in which longstanding ethnic divisions are fueling the violence and civilians are often the targets, not accidental victims, of the fighting.

On Tuesday, the top United Nations human rights official, Navi Pillay, expressed deep concern about “the serious and growing human rights violations” taking place in the country, reporting the discovery of at least one mass grave and the arrests of hundreds of civilians in searches of homes and hotels in the capital of Juba and elsewhere.