"EL-GENEINA, Sudan (Reuters) - Police opened fire and shot tear gas at Darfuri students trying to organize a demonstration on Wednesday against an attack on a village in Sudan's western region which killed 20 people, witnesses said.

Four vehicles with machine guns and two tanks guarded the governor's office as truckloads of soldiers and police raced through the state capital el-Geneina's dusty streets, two days after Arab militia attacked Abu Surooj village in West Darfur state.

U.N. Secretary-General Kofi Annan condemned Monday's attack and urged the government to take action to stop the violence in Darfur. He warned the region was descending into anarchy with increased violence in recent weeks.

Ibtisam Mohamed, 18, and six other girls were at the hospital because they had collapsed after being tear gassed."