A shoplifter caused mayhem in Midtown Saturday — stabbing and seriously injuring a Duane Reade security guard and threatening another person, according to police sources.

The 21-year-old security guard tried to stop the suspected shoplifter from stealing from the drugstore on the corner of West 53rd and 8th Avenue, police sources said.

But the suspect took out a knife and stabbed the male security guard in his arm and the left side of his torso, police sources said.

The suspect also threatened another person outside the store with the knife, according to sources.

Police arrived at the bloody scene just before 4 p.m.

The attack left people in the area shaken.

“I feel concerned and scared. The city has changed so much. We have watch our backs in the stores and on the subway,” said Ermal Gura, 35, of Staten Island, who noted that because of the rise in shoplifting everyone has to  “call customer service just to buy a shampoo.”

They found the male suspect several blocks away with the knife still on him, police sources said.

The security guard was taken to New York-Presbyterian Hospital in serious condition, police said.

Sources said the suspect was arrested and is at the Midtown North precinct.