Fear is a factor keeping more Sudanese from joining unprecedented demonstrations against a regime relying on an extensive internal security network for control, activists and others say.
They say the National Intelligence and Security Service plays a key role in suppressing the demonstrations, not only behind the scenes but also on the street.
"The protests in Khartoum, there's an element of fear in it. People are afraid to go out into the streets because they know what's going to happen", said an activist who was detained by NISS during the demonstrations.
While police almost inevitably fire tear gas at the demonstrators, activists said plain clothes NISS officers actually make the arrests.
"I think NISS is being used specifically as the force to bring down these protests," the activist said. "NISS represents the regime's right arm."
Protests against high food prices began on June 16 at the University of Khartoum.
After President Omar al-Bashir announced austerity measures, including tax hikes and an end to cheap fuel, demonstrations spread to include a cross-section of people around the capital and in other parts of Sudan.
Another round of demonstrations occurred on Friday, which has now become the focus after groups of 100 or 200 initially burned tyres, threw stones and blocked roads in a daily call for regime change sparked by high inflation.
"People are still scared. People are not going out," said a businessman who has not protested but said his friends had been arrested during demonstrations.
Even those who evade the initial security dragnet are being rounded up after the protests, he said, describing the state intelligence system as very strong.
"People are working for them everywhere," the businessman said.
He declined to be named and added that the security service also operates online, where activists are mobilising through Facebook and Twitter.
"It is very extensive indeed. I mean, this is what is keeping the state going," one veteran activist said of the intelligence system.
NISS officers are reputedly much more highly paid than police, who activists say are suffering like most of the population as prices for sugar, fuel and other essentials soar.
The security service "is afraid of police joining the protests," the first activist said.
He added that the security clampdown is "making people like myself think twice" about taking a public stand, "but that doesn't stop us from supporting... from the underground."
Although the numbers of protesters "are not that big yet," he sees the demonstrations growing as Sudan's economic situation worsens.
The Organisation for Defence of Rights and Freedoms, representing political, media, trade union and other activists promoting human rights, said Sudanese police on Friday "attacked" with tear gas and rubber bullets as demonstrators gathered at mosques for weekly protests.
One of the mosques targeted was that of the opposition Umma party in Khartoum's twin city of Omdurman, said the official, asking not to be named.
The European Union said demonstrations have been met with "a violent crackdown" and the United Nations High Commissioner for Human Rights, Navi Pillay, spoke of "very worrying" reports of ill treatment in detention.
Police said they have responded "with a minimum use of force."
Information Minister Ghazi Al-Sadiq has appealed for people "not to allow the rioters to undermine security and stability of the Sudan."
After three weeks of demonstrations nobody has been shot dead by police or the NISS, and some activists say the regime fears creating a martyr.
"The government is cautious. They don't want to go to that extent," said the veteran activist.
In 1964, the death of student Ahmed al-Qureshi sparked the "October Revolution" which brought down the military government then in power after tens of thousands protested.
During an economic crisis in 1985, huge crowds marched in an uprising which led to the bloodless overthrow of president Gaafar al-Nimeiry.
The current unrest -- "unorganised, without an agenda" -- is far from having that kind of outcome for Bashir's 23-year Islamist regime, the businessman said.
"These guys are strong and these guys are smart," he said of the rulers.
The face of the security apparatus became clear last Friday when two agents raided the AFP bureau. They ordered AFP's correspondent to delete pictures of the demonstrations, and detained a freelance photographer for almost 24 hours.
One agent was tall, thin and polite. The other was heavy-set, bearded, and wore a traditional white robe as if he had just attended Friday prayers.
He brandished a silver pistol.
"National security!" he said, after banging on the door.