Sudanese security forces shot dead at least 15 anti-coup protesters and wounded dozens more on Wednesday, medics said, in the bloodiest day since the military's October 25 takeover.

The fatalities -- all in Khartoum, especially its northern districts -- raised to 39 the death toll from unrest since the military seized power, a pro-democracy doctors' union said. Hundreds more have been wounded.

"The day's massacre reinforces our slogans: no negotiations, no partnership, no compromise" with the military, said protest organisers from the Sudanese Professionals Association (SPA).

Demonstrators had taken to the streets across the capital even though telephone lines and internet services had been disrupted since the military took over, AFP journalists reported.

"The people choose civilian rule," demonstrators chanted, also shouting slogans against Sudan's ruler, top general Abdel Fattah al-Burhan.

The demonstrators, mostly young men and women, clapped hands and ululated before the scene turned violent.

As clashes broke out, the security forces also fired tear gas, injuring several more protesters, witnesses said.

Police have denied using live ammunition and state television announced an investigation into the deaths.

The doctors' union said most of the casualties had suffered gunshot wounds to "the head, neck or torso", but added that the demonstrators, undeterred and behind makeshift barricades, kept up their protests.

Demonstrations also erupted in Port Sudan, an AFP journalist said, against the coup which halted a democratic transition that followed the 2019 toppling of longtime dictator Omar al-Bashir.

"It was a very bad day for the protesters," Soha, a 42-year-old protester, told AFP. "I saw a person with gunshot wounds behind me and there were a lot of arrests" in Khartoum.

- Demonstrations after nightfall -

Hundreds remained on the streets after nightfall, especially in northern Khartoum where tear gas fire reached inside hospitals, doctors said. Rallies in other cities dispersed.

Efforts to stem the protests have seen hundreds arrested, including activists, passers-by and journalists. Qatari network Al Jazeera's bureau chief was arrested Sunday and released Tuesday.

The Central Committee of Sudanese Doctors has said security forces have also arrested injured people inside Khartoum hospitals.

The SPA, an umbrella of unions instrumental in the 2019 protests, denounced "immense crimes against humanity" and accused the security forces of "premeditated killings".

US Secretary of State Antony Blinken told Sudan's military the country stood to regain badly needed international aid if it restored the "legitimacy" of civilian government.

Washington has suspended some $700 million in assistance to Sudan since the coup.

"If the military puts this train back on its tracks and does what's necessary, I think the support that has been very strong from the international community can resume," said Blinken.

Prior to 2019, Sudan had been under some form of military dictatorship for much of its modern history.

- US shuttle diplomacy -

Burhan has declared a state of emergency, ousted the government and detained the civilian leadership, derailing a transition to full civilian rule and drawing international condemnation.

He insists the military's move "was not a coup".

US Assistant Secretary of State for African Affairs Molly Phee has been shuttling between the generals and the ousted civilian government in a bid to broker a way out of the crisis.

Phee has called for the reinstatement of ousted Prime Minister Abdalla Hamdok, who is effectively under house arrest.

The few remaining free members of his cabinet continue to describe themselves as the "legitimate" government and refuse to negotiate with the military leaders.

While some of the civilian leaders have been freed since the power grab, new ones have been arrested.

Burhan last week announced a new Sovereign Council, the highest transitional authority, with himself as chief and all nine military members keeping their posts.

Its four civilian members were replaced.

Burhan has also removed a clause in the transitional constitutional declaration that mentions the Forces for Freedom and Change, the key group behind the protests that toppled Bashir.

He has continued to promise elections will go ahead as planned in 2023, reiterating to Phee on Tuesday that his actions aimed to "correct the trajectory of the revolution".