A crazed subway panhandler pulled a knife and threatened a straphanger on a crowded train yesterday before being subdued by a good Samaritan and held until cops arrived, authorities and witnesses said.

The 35-year-old woman boarded the Manhattan-bound N train at around 6:40 p.m. at Queensboro Plaza and began asking for money.

Another woman, 52, began mocking her, unleashing the vagrant’s anger.

“The beggar woman came around and spit at her,” said witness Natalie Arneson. “The heckler spit back and the beggar turned around, charged at her and pulled out a knife.”

“Everybody started running, it was crazy,” said another witness, who asked not to be identified.

Another man, 33, grabbed the panhandler and held her until the train arrived at the Lexington Avenue station.