According to Mr. Hamamm, the activists — from a party that had supported both the original uprising in 2011 and the protests that forced the Islamist President Mohammed Morsi from power in 2013 — were merely chanting the slogan “Bread, Freedom and Social Justice,” when the police arrived in the square.

“Suddenly the security forces fired several tear gas grenades and we were all amazed,” the photographer wrote. “I did not run. I found Shaimaa al-Sabbagh walking beside me, along with couple of protesters who did not run away. Suddenly we found birdshots being fired. I started running and taking photos quickly. When the birdshots stopped, I looked back and I saw Shaimaa al-Sabbagh falling down. I took a couple of photos of her. Then I saw her colleagues trying to make her stand and she did not come up. I saw the police coming and I had to run.”

A spokesman for the Health Ministry, Hossam Abdel Ghaffar, confirmed the young woman’s death to Mada Masr, an independent news site, but suggested that Ms. Sabbagh had been killed during “clashes” between protesters and security forces.

The killing of an unarmed woman, last seen peacefully holding a sign on a sidewalk in downtown Cairo, outraged activists and reminded some of the visceral images showing the fatal shooting of a young Iranian protester, Neda Agha-Soltan, in 2009.