Ch Supt 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 re-usable 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. They have given her a temporary name – Elsa.
“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 Elsa’s life.
“We believe Elsa to have been less than an hour old when she was found. We do not yet know how long she had been there when she was found.
“Our thoughts now turn to the baby’s 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.”
The force is appealing for anyone who may know the mother to contact them urgently.
Tania Iurac, a restaurant manager, said she saw a “commotion” on her walk home from work, adding the baby was left in a “really visible area”.
Iruac, 25, told MailOnline: “I saw a commotion by Greenway park. We saw a white towel on the floor surrounded by six to seven police officers.
“The towel was by a red route clearway sign. But the baby and the plastic bag weren’t there.
“I think the mother wanted the baby to be found. The towel was in a really visible area next to the main road. It was absolutely freezing last night.”
Newham Council have been contacted by the Met with regards to safeguarding the child in the short term.
Mayor Rokhsana Fiaz OBE said: “I was deeply worried to learn of last night’s discovery of a newborn baby girl found wrapped in towels on the Greenway, particularly because of the extremely cold weather we have been experiencing this week.
“That’s why I am so relieved that this precious baby girl is being looked after and doing well in hospital. I am so grateful to the resident who called the emergency services last night, as they have literally saved a life.
“Council officers are supporting the Police with their investigations, and they are working tirelessly to find the mother as all of us are concerned for her safety and wellbeing which is paramount.
“We are here to help, with plenty of local services available to support her with the sensitivity and care she deserves. Our thoughts are with both the mother and baby at what must be an extremely difficult time.”
The Borough of Newham Council was contacted for comment.