The incident happened at Walnut Street Elementary School shortly after 3:30 p.m. during dismissal as students looked on.
Authorities say the guard was helping students cross the street when a man exited his car, chased her down the sidewalk and punched her in the face.
"It's disgusting," said Darby Borough Police Chief Joe Gabe. "She stated an unknown male exited a Nissan Altima and came at her in an aggressive manner, chased her down the street, about a quarter of the way down the block, grabbed her, and struck her in the face with his left fist."
READ MORE | 'It's disgusting': Crossing guard assaulted outside Darby elementary school
'It's disgusting': Crossing guard assaulted outside elementary school
On Wednesday, Pa. State Senator Anthony Williams called the incident an "outrageous and despicable act of cowardice," while offering a reward in the case.
"We will not rest until this individual is brought to justice. The people of Darby will not be intimidated by senseless violence," said Williams. "We stand united in support of our crossing guards and all those who serve our neighborhoods with integrity and care. Anyone with information is urged to contact local law enforcement immediately."
Surveillance video captured the assault. It shows the guard falling to the ground after being hit. She told police that she was knocked unconscious as the man ran back to his car and drove off.
The chief said it's believed the suspect may have been angry about waiting in traffic.
"He may have been upset with having to wait for her to cross children off of the school bus there," said Gabe.
Police said the man was yelling profanities as he drove through the intersection ahead of the attack.
"When he was approaching her, he was yelling more obscenities at her before he grabbed her and struck her in the face," Gabe said.
The crossing guard suffered swelling to her face. Police said she regained consciousness, walked home and then called authorities.
Police are asking for the public's help in identifying the suspect, who was seen driving a gold Nissan Altima. They said the car did not have a license plate but had a paper tag in the window.
Anyone with information is urged to contact police.