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 to the bloody scene just before 4 p.m.

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.