The head of Sudan's opposition political alliance, freed after two days in detention, said on Sunday that the government fears rising popular discontent in the crisis-hit nation.
Farouk Abu Issa, who represents more than 20 opposition parties, told AFP he was released on Saturday after two days of detention by the National Intelligence and Security Service (NISS).
"They are scared because the country is in a real crisis," he said.
"They are scared that I am head of the opposition, and they are scared that the opposition will move" into action.
Issa said he was held after giving a "fiery" speech in support of four dead Darfuri students.
The youths' deaths, following a crackdown on a tuition protest at Gezira University south of Khartoum, last week sparked the largest outpouring of Arab Spring-style discontent since anti-regime protests in June and July.
Issa blamed the deaths on Islamic militia.
"I accused them of killing these boys," he said.
But security agents who detained him made only brief reference to his speech, because no charges were ever brought against him or proper interrogation conducted, he said.
The deaths of the students, originally from the conflict-plagued Darfur region, led to protests last week by hundreds of people who called for "revolution" and the fall of President Omar al-Bashir's 23-year regime.
The Darfur Students Association has said the four went missing -- and were later found dead -- after taking part in a peaceful sit-in which was disrupted by the pro-government student union.
They were protesting for their "right" to free education, the association said.