Forty bodies have been pulled from the River Nile in the Sudanese capital Khartoum.
Following a violent crackdown on pro-democracy protests, opposition activists said on Wednesday.
Doctors linked to the opposition said the bodies were among 100 people believed killed since security forces attacked a protest camp on Monday.
Gathered reports said a feared paramilitary group was attacking civilians. Sudan’s ruling Transitional Military Council (TMC) vowed to investigate.
Residents in Khartoum told the BBC they were living in fear as members of the Rapid Support Forces (RSF) roamed the streets.
The paramilitary unit formerly known as the Janjaweed militia gained notoriety in the Darfur conflict in western Sudan in 2003.
Forty bodies of our noble martyrs were recovered from the river Nile yesterday,” the Central Committee of Sudanese Doctors said in a Facebook post.
An official from the group told the BBC that they had witnessed and verified the bodies in hospitals and that the death toll now stood at 100.
A former security officer quoted by Channel 4’s Sudanese journalist Yousra Elbagir said that some of those thrown into the Nile had been beaten or shot to death and others hacked to death with machetes.
It was a massacre, the unnamed source said. The US state department voiced concerns on Tuesday over the violence to Saudi Arabia a key ally of Sudan’s military rulers.
A department official telephoned the Saudi deputy defence minister Khalid Bin Salman to stress the importance of a transition to a civilian-led government.
On Wednesday, Saudi Arabia called for a resumption of dialogue between “various political forces” in Sudan.
The kingdom affirms the importance of resuming the dialogue between the various parties in Sudan to fulfil the aspirations of the brotherly Sudanese people,” said a statement released by the official Saudi Press Agency.