The UN human rights chief has urged the Sudanese government to avoid "heavy-handed suppression" as demonstrators gear up for mass protests against austerity measures on Friday.
"Tear gas, rubber bullets, live ammunition and other heavy-handed suppression will not resolve the frustrations and grievances of the people, said Navi Pillay, UN high commissioner for human rights, in a statement on Thursday.
Rights groups say scores of people have been arrested since protests against inflation began on June 16 in the Sudanese capital, Khartoum.
Human Rights Watch said the government was also using the protests 'to silence dissenters'.
Sudan has lost billions of dollars in oil receipts since South Sudan gained independence last July, taking with it about 75 per cent of Sudanese crude production.
After President Omar al-Bashir announced austerity measures to tackle the shortfall, including tax hikes and an end to cheap fuel, the protests have spread to include a cross-section of people in numerous locations throughout the capital and other parts of Sudan.
For more than a week, demonstrators in groups of 100 or 200 have burned tyres, thrown stones and blocked roads, while calling for a change in government.
On Thursday, police officers fired tear gas at protesters in the eastern town of Kassala, witnesses said.
"After 23 years of endurance, the Sudanese people have decided to say enough is enough," said an activist from the movement Sudan Change Now.
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In her plea for calm, Pillay also called on Sudan "to immediately and unconditionally release those who have been detained for merely exercising their rights to freedom of assembly and expression".
"Reports of ill treatment in detention are very worrying and must be investigated promptly," Pillay said.
She also urged protestors to ensure, on their part, that no violence or damage to property takes place during the demonstrations.
Pilay's statement comes after Sudan rounded on the US for criticising its handling of the protest movement.
"The USA is not qualified to advise on such an issue because it continues bombing civilians in different parts of the world and it cracked down on demonstrators on Wall Street," foreign ministry spokesman Al-Obeid Meruh said on Wednesday.
He was responding to comments by Victoria Nuland, the US state department spokeswoman, who said that "arresting and mistreating protesters" will not solve Sudan's political and economic crises.
"There have been reports of protesters being beaten, imprisoned and severely mistreated while in government custody.
"We call for the immediate release of those detained for peaceful protest," she said.
On Thursday, riot police stood by as 100 Sudanese lawyers in black legal gowns demonstrated in defence of free speech. They also held signs objecting to high food prices.
"Demonstration is a constitutional right," said one of their banners, while another declared: "Freedom of expression is a legal right."
Eighty legal practitioners in Khartoum held the silent protest by standing outside a courthouse for about an hour, while another 40 carried out a similar action in the capital's twin city of Omdurman.
Last Sunday, lawyers also took to the streets for a protest in El Obeid, capital of North Kordofan state, but some were arrested, witnesses said.
Protests by tens of thousands in 1964 and 1985 helped bring about the downfall of the Sudanese regimes then in office.
Bashir, an army officer who seized power on June 30, 1989, has called the latest protests small-scale and not comparable to the Arab Spring uprisings against regional strongmen over the past year.