The baby was found wrapped in a blanket in sub-zero temperatures by a dog-walker in Newham and is now being cared for in hospital.
Emergency services were contacted at around 9.15pm on Thursday after the baby girl was found at the junction of Greenway and High Street South.
She was not injured in any way and has been given the temporary name of Elsa by hospital staff.
Speaking at a press conference in Newham on Friday, Chief Superintendent Simon Crick, local policing commander for north-east London, said the baby was believed to be black or mixed race.
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.”
Temperatures dropped as low as -4C around London on Thursday night as an Artic blast hit the UK, the Met Office said.
Anyone who knows the mother is urged to contact police by calling 999 and using the reference CAD 6876/18 Jan.