Amateur video of San Francisco police surrounding a suspected knifeman and gunning him down in a hail of bullets went viral Thursday on social media and provoked outrage.The short clip is the latest video posted online showing US police using what appears to be excessive force on suspects -- many of them black and often with lethal effect.The video, taken in broad daylight with a mobile phone by a passenger aboard a bus parked nearby, shows a man wearing a dark jacket walking and leaning against a wall as he is surrounded by at least seven police officers with their weapons drawn.The suspect waves his hands, then puts them briefly in his pocket. A knife is not visible in the footage.As the suspect slowly limps and continues to ignore commands from police, the officers open fire. The person taking the film dives away from the bus window so the footage ends, but about 20 gunshots can be heard ringing out.

Here is the video:(Disclaimer: It is too graphic in nature)