Sudanese opposition leader Sadiq al-Mahdi has been arrested in Khartoum, days after the public prosecutor accused him of blaming the authorities for violence in the troubled Darfur region, his office manager says.

Al-Mahdi, a former prime minister in Sudan's last elected civilian government, is the head of the Umma Party, the most prominent party opposing President Omar Hassan al-Bashir, who ousted him in 1989.

"Two security officers came to al-Mahdi's office at 8.45pm Sudanese time and arrested him without giving any reason," said Mohamed Zaki, al-Mahdi's personal assistant and office manager.

Zaki had no further details about the arrest, which makes al-Mahdi one of the highest-profile figures to be detained in Sudan in recent memory.

The National Intelligence and Security Service (NISS) has the right to detain people for more than four months without judicial review.

Rape and civilian abuses

On Thursday al-Mahdi appeared before prosecutors for questioning after he reportedly accused a counter-insurgency unit of rape and other abuses of civilians in the war-torn Darfur region.

Al-Mahdi was expected to engage in a national dialogue with Bashir's government and other opposition leaders.

The dialogue, which Bashir had called for earlier this year, was expected to ease tension among Sudan's top political parties ahead of parliamentary and presidential polls due next year.

Violence has escalated in the west of Darfur in recent months. Opposition leaders have been accusing government forces of violence against civilians in the region.

Western diplomats and Sudanese security sources estimate that thousands have been killed in clashes between armed groups supporting and opposing the government there since March.

Source: Agencies