Security forces briefly detained the daughter of Sudanese opposition leader Sadiq al-Mahdi, her family said, as authorities continue to press ahead with a crackdown on the weeks-long protests that have spread around the country.

Two security vehicles belonging to the country's powerful National Intelligence and Security Service (NISS) turned up at the residence of Mariam Sadiq al-Mahdi on Wednesday, a day after the agency's chief ordered the release of all detainees held during recent demonstrations.

"I now can confirm that my sister has been released. She had been taken to the headquarters of NISS for questioning," her sister Rabah Sadiq al-Mahdi told AFP news agency.

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Mariam is deputy head of the opposition Umma Party headed by her father - who is Sudan's last democratically elected prime minister and was overthrown by President Omar al-Bashir in a coup in 1989.

Late on Tuesday, 186 protesters were released from detention, hours after intelligence chief Salah Gosh "issued an order to release all detainees held in recent incidents."

Mariam has supported a wave of protests that have shaken cities across Sudan since December 19 when the government decided to triple the price of bread.

Demonstrations have since turned into a nationwide movement calling for al-Bashir's removal. The 75-year-old leader has said, however, that the only way he would step down is through the ballot box.

Rights groups say NISS arrested more than 1,000 people, including protesters, opposition leaders, activists and journalists, as part of its clampdown.

Officials say 30 people have died in violence during the protests, but human rights groups have put the death toll at more than 40.

After nearly a year in exile, Sadiq al-Mahdi returned to Sudan on December 19, the same day as the wave of protests erupted against the government.

"This regime has to go immediately," al-Mahdi told hundreds of worshippers at a mosque in Omdurman after weekly prayers last Friday.

Academics petition for 'peaceful transfer of power'

Separately, around 250 professors from the University of Khartoum protested on campus on Wednesday, demanding a new transitional administration to replace the current one.

About 510 of the university's professors signed a memo calling for the creation of a "sovereign body" to form a new government and oversee a four-year transitional period.

The university educated many of Sudan's leading politicians and has been the scene of protests and unrest throughout the country's tempestuous history.

"The University of Khartoum's role as an academic institution is to find solutions for the peaceful transfer of power," Montasser al-Tayeb, one of the professors, told reporters.