At least one protester was shot dead as security forces confronted crowds marching in Khartoum on Monday demanding the release of prisoners and an end to military rule, medics and a Reuters reporter said.
Elsewhere, thousands returned to streets across Sudan, in some of the biggest demonstrations in nearly a month against an October coup.
Across the River Nile from Khartoum, officers fired teargas to try and disperse thousands of people who approached the disused parliament buildings in Omdurman and got close to a key bridge.
Columns of smoke rose into the sky as demonstrators blocked one of the main streets in Omdurman with stones. Some held giant pictures of protesters killed during previous rallies.
"We won't let the martyrs' killers seize our country. We won't let the military and the (former regime) return again. We are a free and democratic generation," Sara Ahmed, a 19-year-old student, said.
At least 80 have been killed by security forces since the coup, according to medics.
The military and police say they allow peaceful protest, that members of the security forces have had to defend themselves, and that casualties are being investigated.
In Khartoum, separate groups of protesters demonstrated about 2km from the presidential palace amid heavy security, witnesses said.
One unidentified protester was killed by scattered gunshot, said the Central Committee of Sudanese Doctors, a group aligned with the protest movement. There was no immediate comment from the military leadership.
Pictures of rallies in other towns and cities across Sudan were posted on social media.
In recent days security forces have arrested three high-profile civilian figures connected to a task force that was working to dismantle the regime of former President Omar al-Bashir, who was toppled in an uprising in 2019.
"Freedom for the detainees," read a banner unfurled in Omdurman.
An activist lawyers group said last week that more than 100 political detainees were being held without charge.