At least nine people have been killed and several others injured after Sudanese security forces stormed a protest camp in central Khartoum early Monday.
"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 a statement from the main protest group.
Sudan's military has so far made no statements.
The protest outside the military headquarters has been the centre of a campaign to bring a civilian government to power.
Talks between protesters and Sudan's military rulers over the transitional period has ended in a deadlock over who would dominate the top decision-making body.