A fearless Queens mother sprang into action when an alleged stalker tried kidnapping her teen daughter from their Astoria apartment building, according to police and shocking video footage.

A doorbell camera captured the courageous moment Adriana Alvarez sprinted after the deli worker — who’d allegedly become obsessed with her 18-year-old daughter, Lex — as he violently dragged her down the stairs.

Dramatic video shows how the nightmare unfolded just after 9 a.m. on Jan. 23, when Lex is seen returning to their fourth-floor apartment on 43rd Street following a morning walk with their two dogs. Suddenly, a masked man in a camouflage jacket — later identified by cops as George Vassiliou, 25 — emerges from a staircase, where he’d been lying in wait.