A Sudan university dorm caught fire on Tuesday during a clash between protesters and plain-clothed attackers, students said, on the fourth day of unrest sparked by the death of four students.

Students at Omdurman University, in Khartoum's twin city, said they were preparing to demonstrate when government supporters arrived and beat them while police fired tear gas.

During the melee a low-rise residence building burned down, they said, but the cause of the blaze was unclear.

Hundreds of students were then ordered to leave the residential compound which members of the security forces surrounded, an AFP reporter witnessed.

Around the same campus earlier Tuesday, police immediately moved in with batons when about 300 people gathered for a protest in support of the dead youths who were originally from the conflict-plagued Darfur region.

Their deaths, following a crackdown on a tuition protest at Gezira University south of Khartoum, have sparked the largest outpouring of Arab Spring-style discontent since anti-regime protests in June and July.

On Monday hundreds of Sudanese calling for "revolution" rallied outside Khartoum's Nilien University, a day after six people were injured in clashes between protesters and security forces around the city's main bus terminal near Nilien.

Demonstrators have called for the downfall of President Omar al-Bashir's 23-year regime.

The Darfur Students Association said the dead students went missing after taking part in a peaceful sit-in which was disrupted by the pro-government student union.

Gezira University officials say the four students drowned.

New York-based Human Rights Watch said it interviewed witnesses who reported that "government security forces pushed the protesters toward the canal, causing several students to fall in."

In June and July, demonstrations began at the University of Khartoum over high inflation and then spread to involve scattered protests throughout the country.

They petered out following a security clampdown.

The death of student activist Ahmed al-Qureshi in 1964 sparked the "October Revolution" which ended the military regime then in power after tens of thousands protested.