A diplomat in the embassy said the explosion had been so small that no one there heard it or stopped working.

Image Smoke rising after the device exploded outside the embassy. Credit... Adel el-Adawy, via Associated Press

A video that appeared to come from a street-level security camera and was circulated by Egyptian and international news media showed a small explosion knock down a young man with a backpack as he walked by a cement barrier erected near the square. Another video published by state-owned Egyptian news organizations showed police officers removing the man’s pants and escorting him away.

At around 1 p.m., the American Embassy said in a statement that “we are aware of a reported incident on Simon Bolivar Street.” It said people should “exercise caution” and stay away from the area.

About an hour later, the embassy announced that the “police have finished their investigations” and said, “the embassy is resuming normal business.”

The would-be bomber’s exact target and motives could not be determined.

Many Egyptians see the United States as a mainstay of support for President Abdel Fattah el-Sisi, the former defense minister who seized power in the summer of 2013. Washington tacitly accepted his takeover by declining to label it a military coup, and has continued to provide Egypt $1.3 billion a year in military aid.