"There was a crying noise from the bag. She was crying for her life. The baby saved herself," the grandmother said.
She told the Sun Online, "At first I thought it was an animal."
Her son added: "She looked a bit purple. She very cold to the touch. Her forehead and ears looked like they were a bit frosty, she was cold.
"We feel great that we helped save the baby's life. I'm glad we were there or the baby might not have survived too much longer."
Local resident Georgina Player said police came door-knocking at around midnight to tell residents what had happened and to ask for information.
"They said she was only 30-minutes-old when she was found - she even still had her umbilical cord.
"She's at Newham Hospital and I'm going to go there this afternoon to drop some things off for her - babygrows, milk, anything. I feel so bad for the little thing.
"She was left in a towel and plastic bag on the ground, next to the bench - not even on it.
"The police had sniffer dogs trying to find a blood trial but they couldn't, which makes me ask how she left in that state.
"Although it was dark, a lot of people pass through here. You have to walk past to get to the buses.
"I wish she knocked on the door for help but I think she knew someone would find the baby."
The Met Police said officers are growing "increasingly concerned" for the mother's welfare and appealed for her to contact police, her local hospital or GP surgery.