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.”

At a news conference on Monday, the Martin family, their lawyer and supporters said the police were attempting to demonize Trayvon by leaking Mr. Zimmerman’s account to the media.

The most relevant fact in Trayvon’s death, they said, is that Mr. Zimmerman chose to pursue Trayvon, who was unarmed and walking home, despite a police dispatcher’s advice to stay in his car.

“They have killed my son,” Sybrina Fulton, Trayvon’s mother, said tearfully at the news conference. “And now they are trying to kill his reputation.”

Benjamin Crump, a lawyer representing the Martin family, said it was clear from the conversation between Trayvon and his girlfriend just before the shooting that Trayvon was being pursued by a man he did not know, and that he was worried and wanted to get away from him.

The Martin family’s supporters continued to demand Mr. Zimmerman’s arrest and demanded a repeal of Florida’s Stand Your Ground law, which police cite in their inability to arrest Mr. Zimmerman.

“Zimmerman is alive and can say whatever he wants,” said the Rev. Al Sharpton, who attended the news conference along with the Rev. Jesse Jackson. “And Trayvon is dead and can’t defend himself.”

On Monday evening, Mr. Jackson, Mr. Sharpton and the Martin family attended a City Commission meeting, which drew so many people that it was moved from City Hall to Sanford’s civic center. About 500 people sat on folding chairs inside and hundreds more stood outside watching on a jumbo screen, many wearing T-shirts saying, “Do I Look Suspicious?” or “My Hoodie Does Not Mean I’m a Criminal.”

“Something will change because of this,” said Tasha Barnes, 26. “It’s got to change. Police don’t want a riot, and if justice is not made, there will be a riot.”

Mr. Jackson, members of the Martin family and Mr. Sharpton spoke at the meeting, with Mr. Sharpton telling the Sanford city commissioners that the city is risking going down in history as Birmingham and other infamous cities did during the civil rights era

More information came out earlier in the day about Trayvon’s 10-day suspension from high school, a topic the family has been reluctant to discuss but that led to his being brought by his father to Sanford in the days before his death.

Saying that the issue had become a distraction, Mr. Crump announced that Trayvon had been suspended from his Miami high school after school officials found in his bookbag a plastic bag with traces of marijuana inside. Mr. Crump said that he believed at least one other student was suspended in the episode.

“What he was suspended for has no bearing on what happened on Feb. 26,” Mr. Crump said. “He didn’t do anything violent or criminal.”

Later, The Miami Herald reported that Trayvon had been suspended two other times, once for truancy and another time for graffiti. While investigating the graffiti offense, The Herald reported, a school employee found jewelry, a watch and a screwdriver in Trayvon’s backpack.

Trayvon’s father, Tracy Martin, confirmed those two suspensions in a brief interview. But he said that the truancy suspension was because of repeated lateness, not absences, and added that he had never heard anything about jewelry or screwdriver in a bag. “Absolutely not,” he said.