Prosecutors on Wednesday charged a neighborhood watch guard with second-degree murder in the killing of an unarmed US black teenager that sparked nationwide anger amid suspicions it was racially motivated.

"Today, we filed an information charging George Zimmerman with murder in the second degree," Florida state attorney general Angela Corey told a press conference.

"I can tell you, we did not come to this decision lightly," she added after a weeks-long investigation into the February 26 death of 17-year-old Trayvon Martin, adding that Zimmerman -- who is Hispanic -- was in custody in Florida.

Zimmerman stands accused of fatally shooting Martin inside a gated community in the central town of Sanford, as the teen -- wearing a hoodie on the rainy night -- was headed home after buying candy at a convenience store.

Zimmerman described the teen as 'real suspicious,' according to police transcripts. Martin's family and supporters say the teenager may have been the victim of racial profiling.

But Zimmerman's supporters say Martin attacked first, breaking Zimmerman's nose before knocking him to the ground and repeatedly slamming his head against the sidewalk. They insist Zimmerman fired in self-defense.

A controversial Florida law allows the use of deadly force when a person senses a reasonable fear of death or serious injury.

A bond hearing has been scheduled for Thursday, Zimmerman's new defense attorney Mark O'Mara told CNN, adding his client would plead not guilty.

Authorities booked Zimmerman into jail at the Polk Correctional Facility at 8:30 pm on Wednesday (0030 GMT Thursday) on a second degree murder charge.

The case has touched a raw nerve in the US public regarding race and violence -- and O'Mara said his client was 'concerned about getting a fair trial'.

"There has been a lot of information flowing. I think a lot of it has been premature and maybe inappropriate. I don't think a case like this should be tried here," O'Mara told reporters in Orlando, Florida.

Zimmerman "is troubled that the state decided to charge him," O'Mara said. "I think anyone charged with second-degree murder would be scared."

Corey, who was appointed special prosecutor in the case, acknowledged the pressure that her department had been under, saying "we do not prosecute by public pressure or petition. We prosecute based on the facts of any given case as well as the laws of the state of Florida."

Second degree murder is defined as a killing that is not premeditated. A conviction carries a maximum sentence of life in prison, according to US media.

"We simply wanted an arrest -- nothing more, nothing less -- and we got it, and I say thank you," the teen's mother Sybrina Fulton said in a Washington press conference after Corey announced the charges.

"I just want to speak from my heart to your heart, because a heart has no color. It is not black, it is not white, it's red. I want to say thank you from my heart to your heart," she said, breaking down in tears.

The teen's father, Tracy Martin, thanked supporters for their compassion.

"This is just the beginning," Martin said. "We will march and march and march until the right thing is done."

In a statement, Fulton thanked "the millions of people around the world" who signed a petition calling for justice in the case that gathered more than 2.5 million signatures.

"We will continue to fight for justice for Trayvon, not only for our family but to help prevent racial profiling from happening to other teens," she said.

Speaking to reporters earlier in the day, Fulton and Martin shared stories of their pain since their son's death.

"It's been a nightmare for the past 44 days," said Fulton, her voice quivering as she struggled to hold back the tears. "I've been up and down like a rollercoaster. But I know that justice will be served."

"As a parent who loses their child," said Tracy Martin, "it's very difficult to maintain our sanity."

Federal investigators are also continuing a three-week probe of the case.

"If we find evidence of a potential federal criminal civil rights crime, we will take appropriate action," US Attorney General Eric Holder said earlier at the annual convention of the National Action Network, a leading rights group.

Holder said senior Justice Department officials traveled to Sanford to meet with the Martin family and local residents and authorities.

The high-profile case even sparked unusually personal remarks last month from US President Barack Obama.

"If I had a son, he'd look like Trayvon," Obama said, pleading for national 'soul searching' over what he called a 'tragedy'.