A car hit a young boy and his dad on a Westwood street yesterday afternoon — seriously injuring the boy and his father, according to police and neighbors who ran to the aid of the injured.

“It happened in an instant, and people were just lying there,” said Pat McQuillan, who lives two doors down from the intersection of Pond Street and Lakeshore Drive, where two cars collided yesterday afternoon.

One car, a black sedan with its side and rear windows painted with the word “seniors,” hit the pedestrians at about 3 p.m. and the father and son were rushed to Boston hospitals with “varying degrees of very serious injuries,” Westwood police said.

The black car appeared to have crossed the center line and collided with another car, according to police.

Neighbors said the father was walking next to his son and daughter as they pedaled across the street with their dog. The father was struck and was left lying on his side in the road, while the boy, about 6 years old, was under one of the cars. His sister stood nearby unscathed.

“The father was knocked out of his sneakers,” McQuil­lan said.

Tim Seales, who lives directly across from the crash scene, said he heard a loud screech and a bang. He looked outside and saw a little red bicycle wedged between the two cars.

He ran out to the father and told him not to move and that everything was going to be all right. Other bystanders ran to the son under the car and the young daughter standing at the horrifying scene. The dog had injuries to its nose.

The boy’s eyes were blinking and he was breathing. The father, who was breathing but also bleeding, “wasn’t with it, but I kept talking to him, telling him it was going to be OK and that his son was receiving medical care,” Seales said.

The driver of the black car stood to the side of the crash and said she couldn’t see the pedestrians because she was blinded by the sun glare through her front window, Seales said.

“She didn’t know what to do,” Seales said. “She was hysterical. She was crying, hoping and praying for the little boy and the father.”

Two cars back from the crash, a woman who said she was a trauma doctor ran to the scene and started caring for the father and the boy until medics arrived, McQuillan said.

“It was ugly,” McQuillan said. “It was horrible.”

Police Chief Jeffrey P. Silva said in a statement the state police crash reconstruction unit is investigating the accident and that the preliminary cause appears accidental.

“We are hoping and praying for everyone involved and affected by this terrible tragedy,” Silva said.