JACKSONVILLE/SANFORD, Florida (Reuters) - A special prosecutor in Florida charged neighborhood watch volunteer George Zimmerman with second-degree murder on Wednesday in the shooting death of unarmed black teenager Trayvon Martin, a move protesters had demanded for weeks in a racially charged case that has riveted the United States. Zimmerman, who had been in hiding, was under arrest at Florida's Seminole County jail after turning himself in, officials said. His new defense lawyer said he would plead not guilty and that a hearing to establish bail could come as soon as Thursday. Special prosecutor Angela Corey's decision to charge Zimmerman, who is white and Hispanic, came 45 days after the fatal shooting in a quiet gated community in the central Florida town of Sanford. Police had declined to arrest Zimmerman based on his account of self-defense and Florida's controversial "Stand Your Ground" law, setting off civil rights demonstrations across the country. Martin, 17, was on his way back to the home of his father's fiancee when Zimmerman spotted him and considered him suspicious, calling police and defying their advice to give up following the teenager. Zimmerman told police he was walking back to his truck when Martin attacked him, decking him with one punch to the nose. Martin then repeatedly slammed Zimmerman's head against a concrete walkway, Zimmerman's brother and father have said. Zimmerman then pulled out a 9mm handgun he was licensed to carry and shot Martin once in the chest. Witnesses heard screams for help and then a gunshot, sounds that were captured on 911 emergency calls by neighbors. It was unclear who was screaming. Martin's parents said they recognized the voice as their son's. But Zimmerman's brother Robert swore it was George Zimmerman pleading for help. The decision on whether to charge Zimmerman rested with Corey since she was appointed by Florida's governor on March 22. The firestorm of protests had forced Sanford's police chief to step aside and the regularly assigned prosecutor to remove himself from the case, leading to Corey's appointment. The disputed facts surrounding the shooting have been picked apart endlessly by television commentators, while dominating the headlines and reigniting a discussion about guns, self-defense laws and what it means to be black in America. By seeking second-degree murder rather than a lesser charge such as manslaughter, Corey reaffirmed her reputation as a prosecutor who will seek to bring the most serious charge possible. In deciding against arresting Zimmerman, police cited Florida's "Stand Your Ground" law, which allows people to use deadly force when they believe they are in danger of being killed or suffering great bodily harm. Among the first tests in the case will be a hearing in which Zimmerman's lawyer, in order to win immunity under "Stand Your Ground," will have to convince a judge there is a preponderance of evidence to show Zimmerman had a reasonable belief he was in danger. Travyon's parents have appeared on camera repeatedly, demanding justice for their son. "We simply wanted an arrest," Travyon's mother, Sybrina Fulton, told reporters in Washington after Corey's announcement. We wanted nothing more, nothing less. We just wanted an arrest. And we got it. I say thank you. Thank you, Florida. Thank you, Jesus. Zimmerman had been represented by two lawyers, Craig Sonner and Hal Uhrig, who left the case on Tuesday after saying they had lost contact with Zimmerman and reporting that Zimmerman contacted Corey's office directly on Tuesday in an apparent attempt to resolve the case on his own. Sonner and Uhrig said they feared for Zimmerman's emotional and physical well-being from the stress of living in hiding, mostly alone, while television commentators and sports stars demanded his arrest. But his new lawyer, Mark O'Mara, said Zimmerman was fine. "I'm not concerned about his mental well-being right now," O'Mara told Reuters. He seems very lucid. Later, he told reporters he expected Zimmerman to appear at a hearing on Thursday and was hoping a judge would set bail at a level the Zimmerman family could afford. "Let's let the process work," O'Mara said. There are a lot of high emotions, maybe some of them have been salved by the arrest. ... We need to calm this down. It needs to be tried in a courtroom. The shooting received scant media attention at first, but Martin's parents, who are divorced, and their lawyers kept making public calls for Zimmerman's arrest. That eventually led to a media frenzy, outraged online messages by celebrities, and President Barack Obama's comment, "If I had a son, he would look like Trayvon." "There is a little bit of relief," Sanford Mayor Jeff Triplett said, speaking of "the path toward healing" for his city now that community cries for Zimmerman's arrest had been answered. "We call for continued calm in the city of Sanford, its surrounding communities, and across the nation," Triplett said. This is an event that touched many hearts and many lives, and started conversations, but we hope that the atmosphere of civility will continue. Additional reporting by Barbara Liston, Ian Simpson, Chris Francescani and David Adams; Writing by Daniel Trotta; Edited by Prudence Crowther and Peter Cooney