CAIRO — As chaotic antigovernment demonstrations engulfed Sudan’s capital, Khartoum, a young doctor emerged from his hiding place and strode down a deserted street, his hand held high.

An eyewitness said the doctor, Babiker Salama, approached a group of security officials gathered around a truck last week and issued a plea. A protester had been injured and was badly bleeding, he said. Would the officers permit his evacuation?

Exactly what happened next is hotly disputed by Sudan’s president and the protesters seeking his ouster, but the result is not. A gunshot rang out. Dr. Babiker, 27, fell to the ground, grievously injured. An hour, later he was dead.

The death of Dr. Babiker, an idealistic young man from an affluent family, has emerged as a signal moment in a powerful tide of protest that has roiled Sudan over the past five weeks, posing the greatest threat yet to the country’s ruler of 30 years, President Omar Hassan al-Bashir.