(AP) - A prosecutor told jurors in opening statements Monday that George Zimmerman fatally shot Trayvon Martin "because he wanted to," not because he had to, while a defense attorney said the neighborhood watch volunteer shot the teen in self-defense to stop him from smashing Zimmerman's head into a concrete sidewalk. The opposing attorneys squared off on the first day of testimony in a trial that has attracted international attention and prompted nationwide debates about racial profiling, vigilantism and the laws governing the use of deadly force. Defense attorney Don West used a joke in his opening statements to illustrate the difficulty of picking a jury amid such widespread publicity. Knock. Knock," West said. Who is there? George Zimmerman. George Zimmerman who? Ah, good. You're on the jury. Included among the millions likely to be following the case are civil rights leaders the Revs. Jesse Jackson and Al Sharpton, who joined national protests in the weeks before prosecutors filed second-degree murder charges against Zimmerman. The charges came 44 days after the shooting. Zimmerman, 29, who identifies himself as Hispanic, has denied that his confrontation with Martin before the shooting had anything to do with race. His mother was born in Peru. His father is a white American. Martin was black. But just before opening statements began, Martin's parents sent out an urgent plea to their supporters to pray with them for justice, while their family attorney, Benjamin Crump, described the case as clear cut. There are two important facts in this case: No. 1: George Zimmerman was a grown man with a gun, and No. 2: Trayvon Martin was a minor who had no blood on his hands. Literally no blood on his hands. ... We believe that the evidence is overwhelming to hold George Zimmerman accountable for killing Trayvon Martin. Prosecutor John Guy's first words to jurors recounted what Zimmerman told a police dispatcher in a call shortly before the fatal confrontation with Martin: "F------ punks. They always get away. Zimmerman was profiling Martin as he followed him through the gated community where Zimmerman lived and Martin was visiting, Guy said. He said Zimmerman viewed the teen "as someone about to a commit a crime in his neighborhood." And he acted on it. That's why we're here," the prosecutor said. Zimmerman didn't have to shoot Martin, Guy said. "He shot him for the worst of all reasons: because he wanted to," he said. West told jurors a different story: Zimmerman was being viciously attacked when he shot Martin, he said. He was sucker-punched by Martin, who then pounded Zimmerman's head into the concrete sidewalk. "He had just taken tremendous blows to his face, tremendous blows to his head," said West, after showing jurors photos taken by Zimmerman's neighbors of a bloodied and bruised neighborhood watch volunteer. Later, West said it was not true that Martin was unarmed. "Trayvon Martin armed himself with a concrete sidewalk and used it to smash George Zimmerman's head," West said. West also played for jurors the call to a police dispatcher in which Zimmerman used the obscenities. Martin had opportunities to go home after Zimmerman followed him and then lost track of him, but instead the teen confronted the neighborhood watch volunteer, West said. Guy argued, however, that there is no evidence to back up other claims by Zimmerman, including that Martin had his hands over Zimmerman's mouth. Guy said none of Zimmerman's DNA was found on Martin's body. The prosecutor also said Zimmerman's claim that he had to fire because Martin was reaching for his firearm is false since none of Martin's DNA was on the gun or holster. If he is convicted, he faces a maximum sentence of life in prison. On Feb. 26, 2012, Zimmerman spotted Martin, whom he did not recognize, walking in the gated townhome community where Zimmerman and the fiancee of Martin's father lived. The delay in the arrest prompted protests nationwide. The first is a call Zimmerman made to a nonemergency police dispatcher, who told him he didn't need to be following Martin. Both calls were played for jurors by the defense in opening statements. Martin's mother, Sybrina Fulton, left the courtroom before the second call was played. Opening statements were made two weeks after jury selection began. Attorneys picked six jurors and four alternates after quizzing the jury pool questions about how much they knew about the case and their views on guns and self-defense. The prosecution took a little more than a half-hour to make an opening statement. The defense took more than two-and-a-half hours.