The umbrella group leading Sudan's pro-democracy movement has called for a nationwide campaign of civil disobedience, days after a bloody military crackdown left dozens dead.
It called for the campaign to start on Sunday, and continue until a civilian government was installed.
The announcement followed the arrest of three opposition figures who had been involved in mediation efforts.
They had met Ethiopia's prime minister to try to restart peace talks.
However, opposition politician Mohamed Esmat was arrested on Friday soon after his meeting with Ethiopia's Abiy Ahmed, aides said, while Ismail Jalab, a leader of the rebel SPLM-N group, and his spokesman Mubarak Ardol were detained early on Saturday.
Protest leaders have rejected an offer of talks from Transitional Military Council (TMC), saying it cannot be trusted after the bloodshed.
Sudan has been controlled by the TMC since protests led to ousting of long-time President Omar al-Bashir in April.
The military promised a transition to civilian rule, but protesters had maintained a sit-in in Khartoum until security forces swept in on Monday and opened fire.
What is the opposition calling for?
Details of what the action will involve were not immediately available.
"The civil disobedience movement will begin Sunday and end only when a civilian government announces itself in power on state television," Sudanese Professionals Association (SPA) said in a statement.
"Disobedience is a peaceful act capable of bringing to its knees the most powerful weapons arsenal in the world."
What do we know of the arrests?
Armed men detained the two SPLM-N (Sudan People's Liberation Movement North) figures without reason at 03:00 (01:00 GMT), the group told AFP news agency.
Their whereabouts were unknown, as were those of Mohamed Esmat.
On Wednesday, SPLM-N said its deputy head, Yasir Arman, had been 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 Ethiopian prime minister's mediation effort," Khalid Omar Yousef, an opposition alliance leader, told Reuters news agency after Mr Esmat's arrest.
TMC has not yet commented on the arrests.
BBC World Service Africa editor Mary Harper says the latest move suggests that the mediation efforts by Abiy Ahmed have indeed not been taken seriously by the military.
TMC seems emboldened by the political and financial support it has received from Saudi Arabia, the UAE and Egypt, none of which are keen on fully fledged democracy, she adds.
How bad was the violence?
Opposition activists say a feared paramilitary unit, Rapid Support Forces (RSF), killed 108 people in the crackdown, with at least 40 bodies pulled from River Nile in Khartoum on Tuesday. Officials, however, put the figure at 46. Leader of RSF says rogue elements and drug dealers were behind the violence.
RSF, formerly known as Janjaweed militia, gained notoriety for brutal atrocities in the Darfur conflict in western Sudan in 2003.
Khartoum residents have told BBC that they are living in fear in the capital.
A number of women arrested by 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 African Union suspended Sudan's membership "with immediate effect" and warned of further action if power was not transferred to a civilian authority.
Chairman of African Union commission, Moussa Faki Mahamat, called for an "immediate and transparent" investigation into the killings.
Compiled by Olalekan Adeleye
BBC