Sudanese police have raided student dormitories at the main university in the capital Khartoum, arresting and beating hundreds of students, activists say.
The youth activist group Change Now said Friday's raid took place before dawn, adding that more than 350 students had been arrested at the University of Khartoum.
The university has been the scene of student protests since late December. The students have previously staged a sit-in to demand the right to form a student union and to protest against police violence in earlier raids there.
One student said on Twitter they had been subjected to "verbal and physical abuse" and were "beaten".
Another student, Muawia Mohamed, said a Twitter campaign had been launched to donate food to the students until they figure out if they can return to the university.
"Police are said to be all over the streets. Opposition parties have offered to host some of the students," Mohamed said.
"The students have been charged with disturbing the public peace for a silent protest that was supposed to take place on Sunday."
Mariam al-Sadeq al-Mahadi, an pposition member from the Umma Party, said the raid was "an attack on freedom of expression".
A Sudanese lawyer said that before the raid the students had been told to leave the premises but that "at that time students obviously had nowhere to go".
The lawyer, who was not identified, said he would continue to work to get the students out of prison but he added that much would not happen until Monday.
Separately, a Sudanese official said on Thursday Sudan and South Sudan want to have the bulk of their loosely defined and volatile border demarcated as soon as within three months.
The move seeks to ease tensions between the two former civil war foes.
Yahya al-Hussein, a senior government official and member of Sudan's negotiating team, said the demarcation, however, would not include five areas that are still disputed by the two countries.
South Sudan broke off from its northern neighbour in July under a 2005 peace deal that ended decades of conflict.
But unresolved issues such as where to draw the border and how to untangle the oil industry have continued to stoke tensions between the two sides.
Tribal disputes, overlapping territorial claims, rebel fighting and the presence of economically vital oil fields have beguiled attempts to define the exact boundary.
"The two parties have agreed to begin work on drawing the border immediately, and finish work within three months if operating conditions allow for it," Hussein told reporters in Khartoum.
The two sides have agreed on about 90 percent of the border since 2009, Hussein added.
They have been meeting this week in the Ethiopian capital, Addis Ababa, to discuss the border and other sensitive issues such as oil.