* Activists blame authorities for student deaths
* Fire begins in student dormitories (Adds Khartoum student union statement)
By Khalid Abdelaziz
KHARTOUM, Dec 11 (Reuters) - Sudanese police fired teargas to break up a protest by university students in the capital on Tuesday over the deaths of four students from Darfur found dead in a canal last week, witnesses said.
The demonstrations, in their fourth day, have been the most sustained to hit Sudan since a wave of protests against government austerity measures in June, although they have so far failed to muster more than a few hundred students at a time.
Sudan has so far avoided the mass demonstrations that swept leaders from power in Egypt, Tunisia, Yemen and Libya.
On Tuesday, students from Sudan's western Darfur region gathered at Omdurman Islamic University in Khartoum f or another protest when they were attacked by supporters of the ruling National Congress Party, witnesses said.
Police then used teargas and batons to disperse the protesters, forcing them to retreat to the university dormitories, where a fire broke out, the witnesses said.
"We withdrew to the dormitories, and supporters of the ruling party followed us with others wearing civilian clothes, and started burning the dormitories," a student leader said.
The General Students Union of Khartoum State, a group seen as close to the ruling party, blamed the violence on what it claimed was the "kidnapping" of four students by a group loyal to Darfur rebel leader Abdelwahid Nour.
About 100 students were wounded in the clashes, the group said in a statement reported by the state-linked Sudanese Media Centre. There was no immediate comment from the police.
Anger over rising prices and other grievances have triggered smaller protests in Sudan over the past two years, many of them led by students.
STUDENTS FOUND DEAD
The latest round of demonstrations started after four students were found dead in a waterway near Gezira University, in an agricultural region south of Khartoum.
Activists said they disappeared when ruling party supporters attacked students from Darfur demonstrating to demand an exemption from tuition fees, as they say a presidential decree requires.
Gezira state police confirmed they had found student bodies but said there was no sign of violence. The Justice Ministry has said it will set up a committee to investigate the incident.
New York-based Human Rights Watch called on the government to hold those responsible for the deaths to account.
The group quoted witnesses as saying security forces had pushed protesters toward the canal in Gezira, causing some to fall in, although the cause of the four deaths was still unclear. Two students are still missing, it said.
"The murky circumstances of these deaths are fuelling more protests and violence," Human Rights Watch Africa Director Daniel Bekele said in the statement.
Authorities have arrested dozens of people including lawyers and opposition figures since the protests started, the group added.
Demonstrations in Khartoum and other cities against cuts to fuel subsidies last June and July withered under a security crackdown. Eight people were killed in a protest in Darfur's largest city at that time.
Darfur, a vast, impoverished area of western Sudan, has been torn by conflict since rebels started fighting government forces there in 2003, accusing Khartoum of neglecting and marginalising the region. (Writing by Alexander Dziadosz; Editing by Rosalind Russell)