A number of Sudan's bank, airport and electricity workers have been arrested ahead of a strike against the military rule, the main protest group says.
The Sudanese Professionals Association (SPA) says employees are also being threatened by the authorities.
The ruling Transitional Military Council (TMC) has made no comment.
Campaigners have called for ongoing civil disobedience from Sunday, days after a military crackdown left dozens dead in the capital Khartoum.
The military took over after persistent protests led to the ousting of long-time President Omar al-Bashir in April, promising a transition to civilian rule.
But pro-democracy campaigners say the military council cannot be trusted after Monday's crackdown against a sit-in demonstration in Khartoum - and they have rejected an offer of talks.
In a separate development, three opposition figures involved in mediation efforts have been arrested.
What's the latest from Khartoum?
Most offices and businesses remain shut and traffic is light in the city, the BBC's Catherine Byaruhanga in the Sudanese capital reports.
There have been reports of gunfire as security forces maintain their deployments across most parts of Khartoum.
Protest leaders have asked people to stay home and not work as a form of civil disobedience.
They say that demonstrations are no longer possible because of the violent crackdown by the military.
"The civil disobedience movement will begin Sunday and end only when a civilian government announces itself in power on state television," the SPA said in a statement.
"Disobedience is a peaceful act capable of bringing to its knees the most powerful weapons arsenal in the world."
The goal of the strike is to create an effective government shutdown that will make it hard for the military council to govern, our correspondent adds.
What about the opposition arrests?
Opposition politician Mohamed Esmat was detained on Friday soon after his meeting with Ethiopia's PM Abiy Ahmed, aides said.
Meanwhile, Ismail Jalab, a leader of the rebel SPLM-N group, and his spokesman Mubarak Ardol were arrested early on Saturday.
Their whereabouts are currently unknown, and analysts say the arrests suggest that the mediation efforts have not been taken seriously by the military.
On Wednesday, the SPLM-N said its deputy head, Yasir Arman, was arrested at his house in Khartoum. He had returned from exile following the downfall of Mr Bashir.
Mr Esmat and Mr Jalab are both leading members of the Alliance for Freedom and Change, an umbrella organisation of opposition figures, protest leaders and rebel groups.
"This amounts to a practical response from the military council that effectively rejects the Ethiopian prime minister's mediation effort," Khalid Omar Yousef, an opposition alliance leader said.
The TMC seems emboldened by the political and financial support it has received from Saudi Arabia, the United Arab Emirates and Egypt, none of which are keen on fully fledged democracy, says BBC World Service Africa editor Mary Harper.
How bad was Monday's violence?
Opposition activists say a feared paramilitary unit, the Rapid Support Forces (RSF), killed 108 people in the crackdown, with at least 40 bodies pulled from the River Nile in Khartoum on Tuesday.
The Sudanese authorities however, put the figure at 46. The leader of the RSF says rogue elements and drug dealers were behind the violence.
The RSF, formerly known as the Janjaweed militia, gained notoriety for brutal atrocities in the Darfur conflict in western Sudan in 2003.
Khartoum residents have told the BBC that they are living in fear in the capital.
A number of women arrested by the RSF said they were repeatedly beaten with sticks and threatened with execution. They said RSF troops told them to run for their lives, then opened fire. Other victims, they said, were forced to drink sewage water and urinated on.
On Thursday the African Union suspended Sudan's membership "with immediate effect" and warned of further action if power was not transferred to a civilian authority.
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