Amnesty International said it has received "credible reports" that Sudanese security forces have shot dead 37 protesters in a crackdown on demonstrations roiling the country.
Anti-government protests have rattled Sudan since Wednesday after the government tripled the price of a loaf of bread.
The authorities say eight protesters have been killed in the unrest, while Sudan's main opposition leader Sadiq al-Mahdi on Saturday put the death toll at 22.
The rights group said in a statement late Monday it "has credible reports that 37 protesters have been shot dead by the security forces in five days of anti-government demonstrations".
"The fact that the security forces are using lethal force so indiscriminately against unarmed protesters is extremely troubling," said Sarah Jackson, Amnesty's director for East Africa, the Great Lakes and the Horn.
"With dozens already dead, the government must rein in this deadly use of force and prevent more unnecessary bloodshed."
There was no immediate Sudanese government reaction to the Amnesty statement.
Demonstrations have hit a dozen cities across the economically troubled country since the bread price hike last week.
President Omar al-Bashir vowed "real reforms" on Monday in a bid to tamp down a major challenge to his decades-long rule.
Protesters have adopted the slogan used in the 2011 Arab Spring -- "the people want the fall of the regime" -- and attacked offices of the ruling party.
Sudan is mired in economic difficulties including an acute foreign currency shortage and soaring inflation.
Inflation is running at close to 70 percent and the Sudanese pound has plunged in value, while shortages have been reported across several cities including Khartoum.
Demonstrations broke out in January over the rising cost of food, but they were soon brought under control with the arrest of opposition leaders and activists.