The protests sweeping the United States last weekend over the acquittal of George Zimmerman, a neighborhood watch coordinator who was charged with the second-degree murder and manslaughter of 17-year-old African-American Trayvon Martin, is a bitter reminder that racial tensions are still strong four and half years after the country elected its first African-American president.

In Oakland, California, protesters burned flags, vandalized police cars and smashed shop windows.

"And when you think about why, in the African-American community at least, there's a lot of pain around what happened here, I think it's important to recognize that the African-American community is looking at this issue through a set of experiences and a history that - that doesn't go away," President Barack Obama said, calling for calm.