Human rights activists on Thursday accused Sudan's security service of arresting a Darfuri campaigner in Khartoum and holding him without charge for a week.
Abdelmageed Salih was arrested with a friend in the capital's prosperous Khartoum 2 district on Friday, members of the Darfur Democratic Forum, and other activists, told Reuters.
No one was immediately available to comment from Sudan's security services.
Khartoum is highly sensitive about complaints about its human rights record. The government expelled 13 foreign aid groups and closed down three local organisations in March, accusing them of passing information to the International Criminal Court (ICC).
"We were on the side of the road, near a minibus station when security arrested both of us," said Mohamed Ali Ishag, who said he was a friend of Salih and a former member of another campaign group.
"They kept asking me about my involvement with Abdelmageed. When I said it was nothing to do with politics, they let me go on Saturday afternoon. I haven't seen him since."
The Darfur Democratic Forum, one of a number of small Darfuri campaign groups, described Salih as one of its founders and the president of the separate Peace Youth Association.
The Darfur conflict flared in 2003 when mostly non-Arab rebels took up arms accusing Khartoum of neglecting the development of the region.
"The arrest ... represents a clear violation of the international conventions ratified by Sudan," said the Darfur Democratic Forum in a statement. "The DDF calls upon the Sudanese government for the immediate release of Mr Abdelmageed."
Both the Forum and the Association were listed among the signatories of a letter published last year congratulating the chief prosecutor of the International Criminal Court for pursuing a war crimes case against Sudan's president.
International experts say at least 200,000 people have been killed in Darfur, a mainly desert region in western Sudan, while Khartoum asserts the figure is 10,000.
Activists, who asked not to be named, said Abdelmageed had also been involved in investigating reports of sexual assaults in Darfur and the mass arrest and mistreatment of Darfuris in Khartoum after rebels attacked a suburb of the capital in May last year. Sudan's government denied the reports.