A member of the public out walking their dog called police at 21:13hrs on Thursday, 18 January after finding the baby girl in a shopping bag in London.
The baby had been wrapped in a towel.
Officers have completed their work at the scene and the crime scene is now closed.
Police are in contact with local hospitals and the local authority.
Officers are appealing for the child’s mother to come forward.
If you believe you know who the child’s mother is, please call 999 ref CAD 6876/18 Jan.
Chief Superintendent Simon Crick, who leads policing in Newham, said: “Last night we were called by a member of the public out walking their dog, who had discovered a newborn baby in a shopping bag at the junction of Greenway and High Street South, E6.
“Thinking fast, that person kept the baby girl warm until London Ambulance Service paramedics arrived and checked her over before taking her to hospital.
“I am delighted to report that she wasn’t injured in any way and is safe and well in the care of hospital staff.
“I am grateful to the members of the public who stayed at the scene to speak with officers and medics – your actions contributed to saving the baby’s life.
“Our thoughts now turn to the baby mother; we are extremely concerned for her welfare as she will have been through a traumatic ordeal and will be in need of immediate medical attention following the birth.
“Trained medics and specialist officers are ready to support her and we urge her to get in touch by phone or walk in to the nearest hospital or police station.
“If you are the baby’s mother please know that your daughter is well, no matter what your circumstances please do seek help by dialling 999.”