"There is a real risk that the lives of peaceful protesters will be put at risk because of the violent behavior of a few, or because the law may too easily be interpreted by local security authorities in a way that permits them to use excessive force in inappropriate circumstances," Pillay said in a statement.
Egyptian authorities passed a measure requiring protest organizers to give three days' notice of their plans.
"This effectively outlaws spontaneous peaceful demonstrations," she said.
Egyptian news website Ahram Online reported Tuesday authorities used water cannon and tear gas to disperse protesters in downtown Cairo. Protesters have demonstrated against the military's grip on power since the July ouster of Mohammed Morsi, the first democratically elected president in Egyptian history.
Pillay said she was concerned by provisions in the law that permit the use of force by law enforcement officials.
"These include use of tear gas, water cannon, smoke grenades, warning shots, rubber bullets and even live ammunition," she said.
The United States and European Union scaled back military assistance to Egypt this year after the military was suspected of using excessive force to quiet protests.
U.S. State Department spokeswoman Jen Psaki said Monday the law "does not meet international standards and will not move Egypt's democratic transition forward."