An 8-month-old baby was nearly snatched by a stranger on a Manhattan street while being pushed in his stroller, police said, but a bystander intervened to foil the abduction attempt.

A 46-year-old woman was in custody and undergoing psychiatric evaluation Saturday. She hadn't been charged, at least as yet.

A nanny was pushing the stroller when a stranger walked up and tried to grab it from her Thursday afternoon in the Chelsea neighborhood, police said.

"She was saying, 'You're a liar! These are my babies!'" said UPS driver William Marte, who was nearby. He noticed the struggle and stepped in to help.

"I told the lady, 'Let it go — just keep going,'" and she did, he told reporters.

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She was gone when police arrived. They found the suspect after putting up fliers with a sketch.

Yancy Matson, who works nearby, told WABC-TV that he'd seen the woman in the area a number of times.

Marte, 51, said he "did what came from my heart."

"I've got two kids. I did what I was supposed to do," he added.