Horrified bystanders banded together to lift a 5-ton school bus off a pregnant woman pinned underneath, enabling doctors to save her son.

Donnette Sanz, 33, a traffic agent for the city police department, was pronounced dead Thursday, shortly after her baby's birth by an emergency Caesarean section. The 3-pound, 6-ounce boy was in critical condition.

Sanz was walking across a Bronx street around lunchtime Thursday when she was struck by a van that pushed her in front of the bus.

"I ran out of my house to join 10 people trying to lift up the bus," said Cheryl Brown, 47. "At first, we couldn't get it up, so then another 10 people ran over to help, and we got the bus up and the lady out."

The van's driver, Walter Walker, was arrested on charges of criminally negligent homicide and driving without a license, police said.

"My brakes went out," said Walker, 72. "The light turned red, and I couldn't stop. ... I tried to miss her."

Police Commissioner Raymond W. Kelly said Walker had 20 suspensions to his driver's license.

Mayor Michael Bloomberg met with Sanz's husband, Rafael, at nearby St. Barnabas Hospital.

"It's a terrible poignancy that Donnette's son's birthday will now coincide with the day his mother died," Bloomberg said in a statement.

Sanz's sister, Beverly, said the baby was named Sean Michael.