SANFORD, Fla. — In an account given to the Sanford police that was passed on to the state attorney’s office, George Zimmerman, the neighborhood watch volunteer who shot 17-year-old Trayvon Martin on Feb. 26, said that Trayvon had punched him and then repeatedly slammed his head into the sidewalk in the moments leading up to the shooting.

The details were the most thorough yet to be revealed from Mr. Zimmerman’s point of view, and emerged on Monday as thousands were arriving in town to march and attend a meeting about the shooting and the investigation that followed. In the 911 calls that have been released, Mr. Zimmerman is heard deciding, against a dispatcher’s advice, to follow Trayvon, whom he deemed “up to no good.”

In Mr. Zimmerman’s account to the police, he returned to his S.U.V. after he was unable to find him. Trayvon then approached Mr. Zimmerman from behind and they exchanged words. Then, Mr. Zimmerman said, Trayvon hit him hard enough that he fell to the ground — which would explain what Mr. Zimmerman’s lawyer, Craig Sonner, has said was a broken nose — and began slamming his head into the sidewalk.

The account first appeared in The Orlando Sentinel on Monday and was later confirmed by the Sanford police as “consistent with the information provided to the state attorney’s office by the Police Department.”