Beyonce has urged supporters of Trayvon Martin to be inspired by the protests that followed the death of another black teenager, Emmett Till. She is only one of a number of Americans who have drawn a parallel between the two cases - though others point out there are major differences. So who was Till?

After attending an event in New York in memory of teenager Martin, who was shot dead in Florida, the pop singer wrote a message on her website.

"We have made so much progress and cannot allow hatred and racism to divide us," she wrote. "When we all join together, people of all races, we have the power to change the world we live in. We must fight for Trayvon the same way the generation before us fought for Emmett Till."

In the summer of 1955, the 14-year-old Till was far from home when his life ended in a most violent way, apparently for whistling at a white woman....