KHARTOUM (AFP) - Sudan vowed on Sunday to stand firm on its decision to hike fuel prices, despite days of deadly protests and criticism from within the ruling party and hardline Islamic leaders.

Authorities say 33 people have died since the near-doubling of petrol and diesel prices last Monday sparked the worst unrest in the history of President Omar al-Bashir's regime.

Activists and international human rights groups say at least 50 were gunned down, most of them in the greater Khartoum area.

There were no immediate reports of fresh protests early Sunday but an AFP correspondent in Khartoum's twin city of Omdurman said riot police and security forces were on the streets in large numbers.

Information Minister Ahmed Bilal Osman told AFP there was no going back on the fuel price hikes.

"No, it is not possible at all. This is the only way out," he said in a telephone interview.

The minister said authorities had to intervene when crowds turned violent.

"This is not (a) demonstration," he said. "They attacked the gas stations. They burned about 21."

He said the government knew "riots" would occur if the cost of fuel went up but the reduction of subsidies on petroleum will save billions of dollars.

"Our economy cannot tolerate such support," he said. "We have to carry on. We know it is a bit heavy for the people."

Arab-spring style calls for the regime's downfall began after pump prices rose last Monday to 20.80 Sudanese pounds (S$5.88) a gallon from 12.50 pounds, while diesel jumped from 8.50 pounds a gallon to 13.90 pounds.

Fuel prices had already almost doubled last year after a partial lifting of subsidies.

Sudan lost billions of dollars in oil receipts when South Sudan gained independence in 2011, taking with it about 75 per cent of the formerly united country's crude production.

Since then the north has been plagued by inflation, a weakened currency and a severe shortage of dollars to pay for imports.

The country falls near the bottom of a United Nations human development index measuring income, health and education, and it ranked 173 out of 176 countries in Transparency International's index of perceived public sector corruption last year.

Residents say they have been struggling with rising prices for two years.

Yet, until last week when thousands began protesting mainly in the capital Khartoum, the poor had largely failed to take to the streets.

"People have accepted price increases before without much annoyance. But I think it's the overall bleak picture of the economy, and of the country's mismanagement," which finally forced them to voice their discontent, said Rift Valley Institute fellow Magdi El Gizouli.

"These are the protests of the voiceless" he said on Saturday.

Reformers in Sudan's ruling National Congress Party on Saturday told President Bashir that the deadly crackdown on protesters over fuel price hikes was a betrayal of his regime's Islamic foundations.

President Bashir took power in a 1989 Islamist-backed coup.

"The (economic) package that was implemented by the government, and the crackdown against those opposed to it, is far from mercy and justice and the right of peaceful expression," the 31 prominent reformers said in a letter to President Bashir which they made public.

Hardline Islamic religious leaders also issued a statement calling for the government to back down on the price increase.

"We advise the government to turn back to God and provide justice for all Sudanese people, Muslim and non-Muslim," the unofficial group of clerics - who often criticise the regime for straying from Islam - said in a statement late on Saturday.