Eight workers were hurt - one of whom, while not seriously injured, was still in hospital, according to initial reports.
The incidents erupted in several IDP camps around El Geneina in West Darfur at the start of a registration exercise for food distribution conducted by the UN World Food Programme (WFP), with the UN High Commissioner for Refugees (UNHCR) ensuring the inclusion of vulnerable people among the displaced.
"Initial reports from UNHCR staff indicate that as people were lining up to be registered, some groups of young men armed with sticks and stones began attacking aid workers who were supervising the registration," UNHCR spokesman Ron Redmond told a news briefing in Geneva.
"Some of our teams are being escorted back to El Geneina by African Union forces. Some minor injuries have been reported as well as damage to vehicles. But we're still awaiting further details," he added.
UNHCR, WFP and the UN Children's Fund (UNICEF) as well as all non-governmental organizations withdrew from most of the eight camps, including Krindring I and II, Ardamata, Dorti and Abu Zar. The IDP population had requested the registration because there were people residing in the camps who were not IDPs.
There are an estimated 60,000 to 70,000 IDPs in camps around El Geneina. In all, about 2 million people have been displaced in Darfur, scene of fierce fighting over the past two years between rebels, seeking among other things greater economic development, and Government forces and their allied militias.