At least five protesters, including four high school students, have been shot dead after security forces opened fire on a student protest in the central Sudanese city of El-Obeid on Monday (local time), opposition groups have said.

The killings came a day before protest leaders and ruling generals are set to hold new talks on Sudan's transition government following the removal of former leader, Omar al-Bashir, Al Jazeera reported.

Gunfire rang out as teenagers rallied against fuel and bread shortages in the capital of North Kordofan state, residents were quoted as saying, at a time of heightened tensions between opposition campaigners and the military following Bashir's removal.

A key protest group, the Sudanese Professionals Association (SPA), said, "live ammunition" had been used against a "rally of school students" in the central town of El-Obeid.

In a post on its Facebook page, it urged "all citizens and medics" to head to hospitals treating the wounded.

"We call on our people to take to the streets ... to denounce the Al-Obeid massacre, to demand the perpetrators be brought to justice," said the SPA, which had launched the initial protests.

Authorities announced a nighttime curfew in four Sudanese towns following the incident. There was no immediate statement from the ruling military council.

The acting governor of North Kordofan, Mohamed Khidr Mohamed Hamid, told Al-Arabiya TV there had been a "slight friction" between protesters and security forces. He added that he could not confirm who opened fire and a committee would investigate the incident.