Under a neon light in the small interrogation room, Dalia El Roubi's female interrogator scoffed at her request for a lawyer, saying: "You know, this is a dictatorship," Roubi alleges.

Roubi, 36, spent seven days without charge in the custody of Sudan's National Intelligence and Security Service (NISS) after being caught in a dragnet that snared hundreds of people after deadly protests sparked by fuel price hikes in late September.

In an interview with AFP, Roubi, an activist and employee of the World Bank in Khartoum, provided a rare insight into the behaviour of the NISS, a bulwark of the 24-year regime of President Omar al-Bashir who faces increasing public calls to leave power.

Bashir's security forces are believed to have killed more than 200 protesters, many of whom were shot in the head or chest, Amnesty International said.

Authorities have now reported 60-70 deaths and said they had to intervene when crowds turned violent, attacking petrol stations and police.

Eight NISS officers in civilian clothes came to her home on September 30 and told her she had to go in their pickup truck to a nearby police station, Roubi says.

A friend with her was also detained, even though they didn't even know her name, she says.

At the police station, the two women slept on the floor after a "humiliating" interrogation during which, Roubi says, they were called prostitutes.

The following morning, both women were moved to the feared NISS political detention centre, a nondescript complex behind high walls near a bus depot in North Khartoum.

"I was taken with a bedsheet covering my whole body and my face," she says.

'Spreading untrue rumours'

Roubi, her friend, and four other women arrested at about the same time were housed in a women's prison in Khartoum's twin city of Omdurman, but were transferred daily to the political detention centre.

Roubi and the other women were arrested because of things they said on the Internet, Information Minister Ahmed Bilal Osman told AFP.

"They are saying some rumours which are not true, and they are asking the people to go out to demonstration, and in the Facebook they are saying things which are not correct," Osman said.

Blindfolded for the journey to the prison, "you think you're gonna get shot," Roubi says.

They were taken in a minivan with dark windows and curtains, she added.

Interrogation lasted only one hour a day, but they had to wait from noon until midnight.

Roubi says she was not allowed to look at the three NISS women questioning her from behind a wooden desk in a small, cool office.

"I have to look at the wall," she recalls.

The women wanted to know who was in "the movement," and questioned her about "what I was saying in Facebook."

"They kept asking me, 'Do you think this is a Western agenda? Do you think this is an African agenda... are you a spy?'"

They told her to "stop lying" and cooperate or she might be hanged, but "it's not really clear what they actually want," Roubi says.

One officer banged the desk and stood up.

'Quite an eerie silence'

"I’d tell her, 'Maybe it’s better if you sit down.' And then she’d get really angry. At some point she started crying. She complained to her supervisor that I was being disrespectful."

The building's design made it difficult to hear what was happening in other rooms, Roubi said.

"There is quite an eerie silence most times... It’s a very muffled building."

Roubi, who wants peaceful regime change in Sudan, says she was not physically harmed.

Her privileged background and the high-profile nature of her case provided a "protection" not available to the male detainees she saw, squatting with heads down and waiting to be interrogated, she says.

"They sit them towards the wall... and they’re not allowed to lift up their heads. And they're made to drink 7UP or Coke or something to keep them alert."

"There’s hundreds of them, that I saw," and many were blindfolded, Roubi says, adding that NISS officers described them as "African slaves."

Authorities initially said they had detained about 700 "criminals," but Information Minister Osman told AFP late on Tuesday that most have since been freed without charge.

Only about 200 will still be held, to face trial for looting and other crimes, he said.

Roubi says activists have the names of more than 200 prisoners, 10 of whom she knows personally.

"There’s a lot of people that no one knows their names," she says.

Asked about the prisoners' treatment, Osman said there may have been "some inconvenience" during the initial mass round-up, "but there is no torture."

He denied that racist language would be used against detainees.

"No, no, no, no... We are all Africans," he said.