George Zimmerman, charged with second-degree murder in the killing of unarmed black teenager Trayvon Martin, has been released from a Florida county jail on $150,000 bail.

Mr Zimmerman walked out of the John E Polk Correctional Facility in Seminole County moments after midnight local time after posting bail and meeting other conditions set for his release at a pre-trial detention hearing on Friday.

Mr Zimmerman was met by another man and quickly climbed into a white BMW sports utility vehicle that drove off.

He made no comments to a handful of journalists gathered outside the jail.

Under the conditions set by Judge Kenneth Lester Jr, Mr Zimmerman must wear an electronic monitoring device and he may be allowed to leave the state.

He must also observe a dusk-to-dawn curfew and is prohibited from consuming illegal drugs or alcohol or possessing a firearm.

No date has been set for Mr Zimmerman's trial. Due to safety concerns, his whereabouts are expected to remain a closely guarded secret until his next appearance in court.

Mr Zimmerman shot and killed Mr Martin is a gated community in Sanford in central Florida on 26 February in an incident that triggered civil rights protests nationwide and fired a national debate over guns, self-defence laws and race in the US.

Mr Zimmerman has said he shot the 17-year-old in self-defence following a confrontation that occurred as Mr Martin was returning to his father's house in the community after buying candy from a convenience store.

Police initially declined to arrest Mr Zimmerman, citing Florida's "Stand Your Ground" law, which allows people to use deadly force when they believe they are in danger of getting killed or suffering great bodily harm.

The lack of an arrest led thousands to march in protest rallies in Sanford and across the country. The public outrage forced the Sanford police chief and regularly assigned prosecutor to step aside.

At the Friday hearing, Mr Zimmerman apologised to Mr Martin's family.

Governor Rick Scott appointed Angela Corey as special prosecutor. She charged Mr Zimmerman on 11 April.