In this image made from video, Police officers running down a street in Khartoum. Picture: Elmontasir Darwish via AP

Khartoum - At least nine people were killed and many others wounded when Sudanese security forces violently attempted to disperse a long-running mass sit-in, the Central Committee of Sudan Doctors (CCSD) said Monday.

More deaths had been reported by witnesses but still needed to be officially confirmed, the CCSD said in a statement, calling the attack a "massacre."

Earlier on Monday, the CCSD and opposition group the Sudanese Professionals' Association (SPA) reported that five peaceful protesters had been killed after security forces fired live rounds at demonstrators outside military headquarters in the capital Khartoum.

The number of dead had now risen to nine and a large number of protesters were critically injured, the CCSD added.

Security forces were also firing rounds of live ammunition inside Khartoum's East Nile Hospital and were chasing protesters inside the hospital's campus, according to the CCSD.

In this image made from video, civilians walk down a street in Khartoum with plumes of smoke rising in the horizon. Picture: Elmontasir Darwish via AP

The SPA was meanwhile appealing to international medical aid organizations to intervene and help the injured, who it said were trapped in the square, which security forces had now blocked off access to.

The Transitional Military Council (TMC), which has been in power since it ousted long-time ruler Omar al-Bashir in a military coup in April, has denied it ordered an attack on the protesters who are demanding a return to an elected civilian government.

Soldiers had merely targeted an area next to the sit-in, which had been "affecting security," TMC spokesman Sham Eddin Kabashi told broadcaster Sky News Arabic.

In this image made from video, Sudanese forces escort civilian in Khartoum. Picture: AP Photo via AP video

"The concerned authorities decided to move towards this area, to implement security for the society... We did not target the sit-in," said Kabashi. 

The US Embassy in Khartoum, however, said it believed the TMC was responsible for the clash.

"Responsibility falls on the TMC. The TMC cannot responsibly lead the people of Sudan," the Embassy said on Twitter.

"Sudanese security forces' attacks against protesters and other civilians is [sic] wrong and must stop," it added.

Police officers running down a street in Khartoum. Picture: Elmontasir Darwish via AP

The SPA called for a country-wide political strike and civil disobedience campaign in response to Monday's attack.

The group urged citizens to "activate all means of peaceful resistance in order to overthrow the [military] junta and the abhorrent regime. We appeal to all to go out now to the streets in peaceful demonstrations and processions to the squares."

The people of Sudan should block all streets and bridges with barricades and paralyse public life, the SPA said.