Trials began in Sudan on Thursday for a group of 35 people charged with unrest allegedly linked to protests sparked by fuel price hikes last week, a lawyer said.

They are among hundreds rounded up in what the watchdog Amnesty International calls "a crackdown on dissent."

The 35 who appeared in court in Khartoum's impoverished Haj Yousef district included eight youths under the age of 17 and five South Sudanese, said Mutasim al-Haj, one of their lawyers.

They are charged with destruction and creating a disturbance, offences which he said could earn them between three and seven years in prison if convicted.

An initial 48 accused went on trial Wednesday and some were sentenced, the semi-official MediaNews service reported, without giving other details.

In the Haj Yousef case a police officer testified that the accused were detained the day after a demonstration, Haj said.

"No one was arrested during the destruction or the demonstration itself," the lawyer added.

The judge is to issue his verdict on Sunday.

Sudan's government says it has arrested about 700 "criminals" after last week's protests.

But London-based Amnesty said the figures appear to be much higher.

"All indicators are that people are being targeted for arrest for no other reason than they are members of opposition groups, or activists, lawfully exercising their rights to freedom of expression and assembly," it said on Wednesday.

Security forces were believed to have killed more than 200 protesters, many with gunshot wounds to the head and chest, Amnesty added.

Authorities say 34 people died after petrol and diesel prices jumped on September 23 when the government cut fuel subsidies, sending thousands into the streets in the worst urban unrest during President Omar al-Bashir's 24-year rule.

The National Intelligence and Security Service (NISS) announced earlier this week in a full-page newspaper ad that one of its members also died.

Mohammed Rahamatallah Sulieman Ali Zaidan, who was born in 1993, lost his life on September 25 at the entrance to a Khartoum-area bridge, while on duty "against the subversives who targeted the public and private establishments," the newspaper obituary said.

The government said it had to intervene last week when crowds turned violent, attacking petrol stations and police facilities.

Women and children rallied quietly outside NISS headquarters Thursday calling for the release of prisoners held in the crackdown, an AFP reporter said.

The crowd of around 45 carried photographs of the detainees, including social media activist Dalia El Roubi and Amal Habani, a journalist with Al-Khartoum newspaper, the reporter said.

Children held signs reading "Freedom for my mum" while others called for "justice" and "freedom for all."

Roubi, an employee of the World Bank in Khartoum, was taken from her home on Monday. Her husband Abdelrahman Elmahdi told AFP he still had no news about her on Thursday.

He and the couple's 13-year-old daughter attended the demonstration which was organised by families of those detained.

"They didn't try to disperse us," Elmahdi told AFP. "I think we got our message across."

The demonstrators handed over a memorandum for the NISS director demanding the prisoners be released immediately or a police case filed against them, he said.

They asked to know where their relatives are being held and appealed for the prisoners' "humane and dignified treatment," Elmahdi added, vowing further vigils to "make sure our voices are heard and our loved ones come back to their homes."