All at once, four security guards descend on the two brothers, shoving them through the front doors of the Egyptian Mission to the United Nations.

Through the glass doors, the glint of a metal chain is briefly visible in one of the guard’s hands. He pulls back his arm and swings wildly at the older of the brothers. Then he does it again.

“Bring somebody,” a voice calls out urgently. “They’re beating the kids with the chain.”

The scene, captured in videos recorded by one of the brothers and an activist, unfolded on Aug. 20 at the Egyptian diplomatic outpost on East 44th Street in Manhattan. The brothers, Yasin El Sammak, 22, and Ali Elsamak, 15 — who spell their last names differently — had been on the street outside the building as part of a protest demanding that Egypt do more to allow humanitarian aid to flow into Gaza.

They had come to record the event, according to their father, Akram Elsamak. But he said they were soon confronted by the guards, who dragged them into the front lobby, beating the older brother to the ground and choking and manhandling the teenager.

New York police officers who were called to assist arrested the brothers, charging Mr. El Sammak with assault and Ali with assault and strangulation, even as the snippets of video and two separate accounts of the events do not indicate that either assaulted the guards.