Sudan's state security arrested opposition leader and ex-premier Sadiq al-Mahdi after he reportedly accused a counter-insurgency unit of rape and other abuses of civilians in Darfur.
Mahdi is the head of the Umma Party, the most prominent party opposing President Omar al-Bashir, who ousted him in 1989.
A Umma official said agents arrived in two trucks and several cars to arrest Mahdi, one of the highest-profile figures to be detained in Sudan in recent years.
The National Intelligence and Security Service (NISS) has the right to detain people for more than four months without judicial review.
Earlier on Saturday Mahdi had addressed thousands of people in Gezira state, south of the capital Khartoum.
He told them it was time to end the country's silence over activities of the security service, witnesses said.
A government source said that Mahdi was being held under an order from prosecutors because he had continued to repeat his allegations against the Rapid Support Forces (RSF).
On Thursday, the Umma leader appeared before prosecutors for questioning over the allegations he first made about the RSF at a press conference earlier this month.
Newspapers reported that NISS, which has authority over the RSF, filed a complaint accusing Mahdi of threatening public peace, and other crimes.
At a news conference in Khartoum on Wednesday, commanders of the RSF denied their force had looted, raped or committed arson.
"All the allegations against us are lies," an angry Mohammed Hamdan Dalgo, the unit's field commander, shouted.
Mahdi's detention comes as Umma and some other opposition parties engage in a "national dialogue" with Bashir.
A senior opposition politician has said that Umma was a main focus of the dialogue process that might lead to a new, coalition government.
The politician said Bashir was pushing for "a real change" because he realises the country is "collapsing".
The security service was resisting the dialogue process, the politician said.
A political scientist, El Shafie Mohammed El Makki, has said that even if Bashir himself was serious about reform, "not all the people in his party are for what is going on. You have to understand this".
The Bashir regime, which took power 25 years ago in an Islamist-backed coup, has faced mounting challenges since the separation of South Sudan three years ago
Inflation has soared and the Sudanese currency has sunk. Wars and unrest have spread to about half of the country's 18 states, and internal divisions have surfaced in Bashir's National Congress Party.
Repression peaked in September when thousands called for the regime's downfall after fuel price increases.
Security forces were believed to have killed more than 200 protesters, many with gunshots to the head and chest, Amnesty International said.
Those demonstrations made clear the urgent need for reform, which Bashir addressed in January when he appealed for a broad national political dialogue and "renaissance" focused on peace.
Since then, in a tenuous political opening, some opposition political rallies have drawn thousands of people without interference by the security forces.
But youth activists have continued to be arrested.
Sixteen opposition parties have refused to join Bashir's dialogue unless certain conditions are met - including the abolition of laws restricting freedom.
Mahdi's government was overthrown in the 1989 coup which brought Bashir to power.
More recently the Umma leader has been widely regarded with suspicion by Sudanese, partly because one of his sons is a Bashir adviser.
The president and Defence Minister Abdelrahim Mohammed Hussein are both wanted by the International Criminal Court for alleged war crimes in Darfur.