The US justice department has begun an investigation into the fatal shooting of an unarmed black teenager in Florida by a neighbourhood watch captain.

The announcement came after a day of protests calling for the arrest of the Florida volunteer, George Zimmerman.

Zimmerman, 28, claims he shot 17-year-old Trayvon Martin last month in self-defence during a confrontation in a gated community in Sanford, Florida.

Zimmerman spotted Martin as he was patrolling his neighbourhood on a rainy evening last month and called the police emergency dispatcher to report a suspicious person. Against the advice of the emergency dispatcher, Zimmerman then followed Martin, who was walking home from a convenience store with a bag of Skittles sweets in his pocket.

The justice department said in a statement late on Monday that the FBI and the US attorney's office will join in the agency's investigation.

"The department will conduct a thorough and independent review of all the evidence and take appropriate action at the conclusion of the investigation," the agency said.

The case has garnered national attention. Civil rights activist Al Sharpton is expected to join Sanford city leaders in a Tuesday evening town hall meeting to discuss how the investigation is being handled with residents. The justice department said its community relations service will also be in Sanford this week to meet with authorities, community officials and civil rights leaders "to address tension in the community".

Earlier on Monday, college students around Florida rallied to demand Zimmerman's arrest, though authorities may be hamstrung by a state law that allows people to defend themselves with deadly force.

Prosecutors may not be able to charge Zimmerman because of changes to state law in 2005. Under the old law, people could use deadly force in self-defence only if they had tried to run away or otherwise avoid the danger.

Under the new law, there is no duty to retreat and it gives a Floridian the right "to stand his or her ground and meet force with force, including deadly force," if he feels threatened.

Students held rallies on the campus of Florida A&M University in Tallahassee and outside the Seminole county criminal justice centre, where prosecutors are reviewing the case to determine if charges should be filed. The students demanded Zimmerman's arrest.

"I don't think a man who exited his vehicle after the 911 dispatcher told him to stay inside the car can claim self-defence," Carl McPhail, a 28-year-old Barry University law school student, said at the Sanford rally.

The 70 protesters at the Sanford rally chanted "What if it was your son?" and held posters saying, "This is not a race issue." Many carried Skittles.

Martin's parents and other advocates have said the shooter would have been arrested had he been black.

"You would think that Sanford is still in the 1800s claiming that this man can call self-defence for shooting an unarmed boy," restaurant owner Linda Tillman said.

Florida congresswoman Corrine Brown along with members of the Congressional Black Caucus and the Congressional Hispanic Caucus, had asked the justice department to review the case, and White House spokesman Jay Carney said earlier on Monday during a briefing that officials there were aware of what happened.

"Our thoughts and prayers go out to Trayvon Martin's family," Carney said at the time. "But obviously we're not going to wade into a local law enforcement matter."

Florida Governor Rick Scott has directed the state department of law enforcement to help local authorities in their investigation. The governor said in a memo to department Commissioner Gerald Bradley that the circumstances surrounding the death "have caused significant concern within the Sanford community and the state".

Prosecutors can have a hard time making a case if there is no one else around to contradict a person who claims self-defence, said David Hill, a criminal defence attorney in Orlando. So far, Sanford police have said there is no evidence to contradict Zimmerman's claims.

"If there is nobody around and you pull a gun, you just say, 'Hey, I reasonably believed I was under imminent attack. Hey, sorry. Too bad. But you can't prosecute me,'" Hill said, somewhat tongue-in-cheek.

Gun control advocates said the case is emblematic of permissive gun laws in Florida, which was among the first states to allow residents to carry concealed weapons. Florida was the first state to pass a "Stand Your Ground" law, which has been dubbed a "Shoot First" law by gun control advocates.

Currently, about half of all US states have similar laws, said Brian Malte, legislative director of the Brady Campaign, which describes itself as the nation's largest organisation dedicated to the prevention of gun violence.

"It's coming to dangerous fruition," Malte said. "There are more states like Florida."

The "Stand Your Ground" law's legislative sponsor, Florida Representative Dennis Baxley, said it was not written to give people the power to pursue and confront others.

"That's not what this legislation does," said Baxley, a Republican.

"Unfortunately, every time there is an unfortunate incident involving a firearm, they think it's about this law, and it's not."