The newborn was discovered wrapped in a towel at the junction of Greenway and High Street South, E6, at 9.13pm on Thursday by a member of the public who ensured the baby was kept warm.
She was checked over by London Ambulance Service paramedics and was not found with any injuries and is currently being looked after by hospital staff.
Restaurant manager Tania Iurac, 25, who lives nearby, told the PA news agency: “My friend came home from work at around 11pm and she told me a lot of police officers were outside with a dog and the area was closed.
“I was worried. We went outside and asked the police for more information. I saw a white towel on the floor. It was clothing. It did not have blood on it or anything. The baby was not in there.
“There was… I would say, six to eight police officers around. There were a lot of people asking questions. I am just shocked.”
She said she did not know the identity of the mother.
Chief Superintendent Simon Crick, who leads policing in Newham, said: “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 into 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.”
Police have completed their work at the scene and are in contact with local hospitals and the local authority. They have urged the child’s mother to come forward.
This comes as temperatures dropped below freezing overnight and reached -6C in London, with an Arctic blast causing snow and ice to hit the UK.
Anyone with information is asked to contact the Metropolitan Police on 999, quoting reference CAD 6876/18 Jan.