Obama urges 'soul-searching' after black teen's killing

President Barack Obama urged Americans to do some "soul searching" today after the killing of an unarmed black teenager by a neighbourhood watch guard that has sparked outrage across the nation.

"If I had a son he would look like Trayvon," Obama, the country's first African American president, told reporters, as he called the fatal shooting of 17-year-old Trayvon Martin a "tragedy".

"I think all of us have to do soul searching to figure out how does something like this happen and that means we examine the laws and the context for what happened."

Up to 20,000 people demanding justice rallied last night in the Florida town of Sanford where Martin was shot dead by volunteer neighbourhood watch guard George Zimmerman in late February in a gated community.

The mostly African-American crowd demanded the arrest of Zimmerman, who claims he acted in self-defence after a confrontation with the teenager. Zimmerman has been neither detained nor charged with any crime.

Protesters have claimed that Martin's killing is just the last example of racial profiling and unjust treatment of blacks by the country's criminal justice system.

An online petition has gathered more than a million signatures following Martin's fatal shooting on February 26, and the Department of Justice has launched an investigation into the events.

The FBI, the state attorney's office and the Florida Department of Law Enforcement are also investigating the case.

Local police say they believe Zimmerman cannot be prosecuted thanks to a Florida law that lets state residents use lethal force in self-defence.

"Obviously, this is a tragedy. I can only imagine these parents are going through. And when I think about this boy I think about my own kids," Obama said, addressing reporters in the White House Rose Garden.

He said Martin's parents "are right to expect that all of us as Americans are going to take this with the seriousness it deserves and we'll get to the bottom of what happened.