But authorities were still holding 51 southern students accused of rioting in the Juba University in the capital over the weekend. U.N. human rights chief Louise Arbour has said security services in Sudan work in a "climate of impunity".

Amnesty said credible sources told them the students were being interrogated and tortured in detention centres known as "ghost houses" in the capital Khartoum.

In a statement the head of the Sudanese non-governmental organisation SUDO, Mudawi Ibrahim Adam, said his five employees, arrested on Monday in the Darfuri town of Ed-Daien, were released after Amnesty's appeal.

"The governor received the first faxes sent by Amnesty International -- very soon thereafter the five were released. The pressure exerted ... has really worked," he said.

Sudan's state security often crackdown on NGOs working in the country, and arrested two senior officials of the international Medecins Sans Frontieres (MSF) last year accusing them of spying and publishing false information.

MSF had published a report based on their doctors treating around 500 rape victims in Darfur in just a few months. The government denies widespread rape in the area which has been embroiled in a civil war for three years.

Amnesty says Sudanese aid workers and rights activists are harassed much more than internationals and often their fate goes unnoticed.

Tens of thousands of people have died and more than 2 million have fled their homes in the arid region during violence that Washington calls genocide.