Sudanese security forces have used live ammunition in a major operation to disperse a protest camp in central Khartoum, witnesses and Arab television stations reported.
A medical association affiliated to protesters said at least two people had been killed and many injured in the attack, which is still in progress.
The news of the crackdown sparked sporadic unrest around Khartoum. Thousands of protesters were reported to have blocked roads with stones and burning tyres in Omdurman, the twin city neighbouring the Sudanese capital.
Smoke was seen rising from several locations and at least one bridge across the Nile was reported to have been blocked by burning tyres.
The sit-in has been the centre of a campaign to bring democratic reform to Sudan, which has been ruled by a military committee since the fall of the dictator Omar al-Bashir in April.
Witnesses reported that security forces belonging to the feared Rapid Support Forces had surrounded and entered hospitals, beating medical staff and injuring protesters.
“The protesters holding a sit-in in front of the army general command are facing a massacre in a treacherous attempt to disperse the protest,” said the Sudanese Professionals Association, the group that spearheaded nationwide protests that started in December.
Live images broadcast by Arab television stations showed tents used by the protesters on fire, as other demonstrators ran away from the scene. There were reports of heavy and sustained gunfire at the protest site, which is in front of the defence ministry.
A witness living in the Burri neighbourhood in east Khartoum said he could “hear the sound of gunfire and I see a plume of smoke rising from the area of the sit-in”.
Another resident of the area, which is near the sit-in, said he had seen forces in police uniforms trying to expel the demonstrators.
The SPA said it amounted to a “bloody massacre”, and called on Sudanese people to take part in “total civil disobedience” to topple the military council and for people for take to the streets to protest.
The Sudanese military ousted Bashir in April after months of protests against his 30 years in office. Thousands of protesters camped outside the ministry, first demanding that military rulers oust Bashir and then that they hand over power to civilians.
Though consensus on the broad outlines of a deal to install a civilian government has been reached, protracted negotiations between a coalition of pro-reform groups and the military have foundered on the question of who would dominate the top decision-making body during an interim period.
The coalition called for peaceful demonstrations and protests across the country in response to the crackdown.
Ifran Siddiq, the British ambassador in Sudan, said he was extremely concerned by the heavy gunfire he had heard from his official home in Khartoum and the reports that Sudanese security forces were attacking the protest sit-in site.
“[There is] no excuse for any such attack,” he said on Twitter.
Monday’s operation appeared to be spearheaded by the RSF, which have been accused of systematic human rights abuses. They are led by Mohamed Hamdan Dagalo, known as Hemedti, who also serves as deputy head of the transitional military council, the country’s governing committee.
This is a developing news story, please check back for updates.