John Garang, a former rebel leader who became Sudan’s vice president three weeks ago, has been killed in a helicopter crash, raising serious concerns about a peace process that ended a 21-year civil war and efforts to bring stability to Africa’s largest country.

After his death was announced on Monday, thousands of southern Sudanese rioted in Khartoum, clashing with police and looting shops. Similar unrest was reported in Juba and other southern towns in what were government-controlled areas, indicating the volatile situation in a country blighted by decades of conflict and misrule.