Hamilton Street Bridge Allentown
The 19-year-old mother tossed her baby from the Hamilton Street Bridge 16 meters down into the Lehigh River KTLA/YouTube

A young mother hurled her year-old son off a Pennsylvania bridge, according to police, then leaped in after the baby. Both survived thanks in part to quick calls to police by witnesses.

Police characterised the incident at the Hamilton Street Bridge in Allentown as an attempted murder-suicide by the 19-year-old mother.

She was seen pushing her baby across the bridge in a carriage, when she stopped suddenly, picked up the boy, kissed him, then threw him over the side to plunge 16 metres (52 feet) into the shallow Lehigh River, according to witnesses. She then climbed over the railing and jumped.

The young woman was rescued first just five minutes before the baby boy was located and saved 640 metres (700 yards) downstream. Responding officers performed mouth-to-mouth resuscitation on the baby before he was rushed by ambulance to hospital. He is in a critical condition but expected to survive.

"The officers basically saved this baby's life," said an Allentown police captain, calling the work "spectacular."

The mother, who suffered minor injuries, has been charged with attempted murder, aggravated assault and reckless endangerment, reports NBC. She remains under guard at hospital.