A dog walker discovered the baby just before 9.15pm on Thursday. The baby is now safe and well in hospital.
The baby, who had been wrapped in a towel, was found at the junction of Greenway and High Street in Newham by a dog walker believed to have heard her crying.
Members of the public helped paramedics and police who were scrambled to the scene with fears over the physical state of the tot on one of the coldest nights of the year.
Police revealed at a press conference on Friday that she is believed to have been less than an hour old when she was found. She has been given the temporary name Elsa by hospital staff.
Chief Superintendent Simon Crick, local policing commander for north-east London, said the child was not injured in any way.
Mr Crick said: “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."
He said he believed "there were some blankets left with the child” when she was left in the bag.Making an appeal for the mother to come forward, Mr Crick said: “We are extremely concerned for her welfare as she would 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.”
Mr Crick said earlier: "Last night we were called by a member of the public out walking their dog, who had discovered a new-born 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."
Restaurant manager Tania Lurac, 25, who lives nearby, said: "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.
A Stratford resident who has been living in the area for more than 30 years said the area is quiet and mainly used by people passing through.
The resident, known only as Su, told the Standard: "Generally speaking people use this to jog, to cycle, perhaps to go for a stroll. It’s just a through road.
"I’ve never seen anything like that I’m just shocked that, in this weather in particular, someone would leave a baby out here.
"To leave a baby here the assumption is that that baby is not going to be found. It’s not frequented enough.
"It’s pitch black at 4 o’clock, it’s winter.”
A dog walker who uses the greenway every day said they had never heard of a “crazy” situation happening in the area.
They told the Standard: "I would have panicked and called the emergency services. That is crazy."
If you believe you know who the child's mother is, please call 999 ref CAD 6876/18 Jan.