West told jurors a different story: Martin sucker-punched Zimmerman and then pounded the neighborhood watch volunteer's head against the concrete sidewalk, and that's when Zimmerman opened fire.
Showing the jury photos of a bloodied and bruised Zimmerman, the defense attorney said, "He had just taken tremendous blows to his face, tremendous blows to his head."
West said the story that Martin was unarmed is untrue: "Trayvon Martin armed himself with a concrete sidewalk and used it to smash George Zimmerman's head."
The prosecutor, however, disputed elements of Zimmerman's story, including his claim that Martin put his hands over Zimmerman's mouth and reached for the man's gun. Guy said none of Zimmerman's DNA was found on Martin's body, and none of the teenager's DNA was on the weapon or the holster.
But West said that doesn't prove anything, arguing that crime-scene technicians didn't properly protect Martin's hands from contamination.
Two police dispatch phone calls that could be important evidence for both sides were played for the jury by the defense. Martin's mother, Sybrina Fulton, left the courtroom before the second recording, which has the sound of the gunshot that killed Martin.
The first was a call Zimmerman made to a nonemergency police dispatcher, who told him he didn't need to be following Martin.
The second 911 call, from a witness, captures screams in the distant background from the struggle between Zimmerman and Martin. Martin's parents said the screams are from their son, while Zimmerman's father contends they are his son's.
Circuit Judge Debra Nelson ruled last weekend that audio experts for the prosecution won't be able to testify that the screams belong to Martin, saying the methods used were unreliable.
On Monday, one of the first witnesses for the prosecution was a custodian of police dispatch calls. During the witness' testimony, prosecutors started playing police calls Zimmerman had made in the months before he shot Martin. The defense objected, arguing the calls were irrelevant.
The judge said she would address the matter Tuesday and sent the jurors to the hotel where they are being sequestered for the duration of the trial, which could last several weeks
Other witnesses who testified Monday included a convenience store clerk and the 911 dispatcher who took Zimmerman's call when he was following Martin. Martin had gone to the convenience store to buy Skittles and a can of iced tea.
The 911 dispatcher, Sean Noffke, testified that he had advised Zimmerman not to follow Martin.
Randy McClean, a criminal defense attorney in Florida with no connection to the case, called the prosecution's opening statement "brilliant" in that it described Zimmerman's state of mind. But he described the knock-knock joke as less than stellar.
"If you're defending your client for second-degree murder, you probably shouldn't start your opening with a joke," McClean said.