Hundreds of supporters of detained former Sudanese prime minister Sadiq al-Mahdi rallied on Friday despite a show of force by police and security agents.
The National Intelligence and Security Service (NISS) arrested Mahdi, chief of the opposition Umma Party, on May 16 after he reportedly accused a counter-insurgency unit of abuses against civilians.
Mahdi, a descendant of Sudan's legendary Islamic reformer known as "the Mahdi", is also a religious leader revered by followers in his Ansar ("Partisans of Islam") movement.
Ansar is a key component of Umma.
"The voice of Sadiq is the voice of the people!" his followers shouted after hundreds gathered for Friday prayers at Ansar's Wad Nubawi mosque in Khartoum's twin city of Omdurman.
Dozens of vehicles from the riot police, NISS and police special forces were on standby in the area.
Mahdi's followers then moved to the nearby Umma headquarters where they carried on their rally.
On Thursday, the US embassy expressed "deep concern" over the detention of Mahdi and urged Sudan to respect freedom of expression.
Mahdi reportedly accused the Rapid Support Forces (RSF), which are under the authority of NISS, of rape and other abuses of civilians in the Darfur region of western Sudan.
RSF commanders denied that their unit had raped, looted or committed arson.
Observers have said Mahdi's arrest undermines talks launched by President Omar al-Bashir to resolve the multiple crises gripping the impoverished, war-torn country.