Chief Const. Jim Chu said a man summoned police officers directing traffic across the street at 11 p.m. Wednesday, saying a woman had just given birth to a baby.
He said two officers found the woman immediately and saw her holding a tiny infant tightly by the neck and the head as she walked away.
The woman's water broke and she gave birth to a second baby, Chu said, adding the officers were alarmed to see her squeezing that baby's neck as well and grabbed both infants to protect them.
"By intervening and removing the infants they saved the babies' lives, in my view," he said Friday. "They had to act. The babies were in grave danger."
Chu said paramedics and firefighters arrived on the scene and the five officers found themselves in a chaotic situation as the woman refused their help and pleas to go to hospital, saying she didn't trust doctors.
"She screamed, she said, `Get away. I don't know who you are,' despite the officers being in full uniform."
On Thursday, the police department announced the birth of the babies, a boy and girl each weighing less than two kilograms, but excluded the sad details Chu recounted Friday.
He said police wanted the public to know the truth about what happened but first had to get legal advice because information about child apprehensions isn't typically released.
"This woman has, for some time, suffered from mental disorders and we have encountered her before," Chu said, adding the babies have been turned over to the Children's Ministry.
He said the infants appeared to be OK when they were taken to hospital on Wednesday night but he could not provide any updates about them or their mother's condition.
"We're always concerned, as a police department, about the mentally ill and the services that are provided. I used to say we're the mental-health agency of second resort. Now I say we're the mental-health agency of first resort."
The Vancouver Police Department has issued two reports since 2008 about the challenges of dealing with mentally ill individuals, some of whom are chronic offenders battling drug addiction.
A report released last year was titled Policing Vancouver’s Mentally Ill: The Disturbing Truth.
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