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.

"Some of the boys at the school wanted to go to the demonstration and the headmaster refused to let them go," she said. She lay on plastic sheeting on the floor of the emergency room in the hospital, struggling for breath.

"That's when they opened fire with the tear gas bombs."

Next door, 22-year-old student Mohamed Osman Mokhtar was being operated on. He had been shot in the leg.

"Mohamed is finished, he's finished," wailed the young man's uncle. "This was no accident. They shot many times at the school." His mother and sisters cried silently outside.

WORSENING SECURITY

Many of the Abu Surooj dead and wounded were also brought to el-Geneina, about 30 km from the border with Chad.

On Tuesday Darfuris stoned a policeman to death in the town's market after authorities opened fire and flew an attack helicopter over the crowd to disperse them.

Security in West Darfur has deteriorated in recent months, hindering aid access to what the United Nations last year called the world's worst humanitarian crisis, with tens of thousands killed and more than 2 million forced from their homes.

Non-Arab rebels took up arms in early 2003 saying they were marginalised by the central government. The United Nations says Khartoum mobilized Arab militia to fight the rebels.

The militia stand accused of a widespread campaign of rape, looting and killing, which the United States called genocide.

Peace talks in the Nigerian capital Abuja are ongoing but the chief mediator Salim Ahmed Salim on Wednesday said in a statement he was "utterly outraged" by the Abu Surooj attack.

"He unequivocally condemns the unwarranted brutal killings of numerous innocent civilians including women and children, and the destruction of their homes and property by armed militia," it said.

Government officials declined to immediately comment.

Tensions have increased after Chadian army deserters attacked the border town of Adre three days ago. Chad said they chased the dissidents back into Sudan, killing 300 of them, and blamed Sudan's government for the attack.

Sudan denies any support for the Chadian deserters.