Intro for sci-fi / horror movie

A gang of friends are walking in a city. One of the friends drops a little behind the others, and sees some kids lying back down just next to each other on a patch of grass (in a park or similar). A small ball (baseball?) comes rolling and cuts straight through the kids bodies. Some mysterious men come and wrap the remains of the kids bodies in some kind of plastic and hurls them up into the sky, and the bags just whooshes of with extremely high speed, and the men disappear. All in all it takes just a couple of seconds. The guy is shocked, and runs towards his friends. Then he see a football (soccer ball) come rolling toward his friends. He shouts to them to not touch it. They look at him and laugh, as one of the friends stops it with his foot. Unharmed. A second ball comes rolling and a second friend laughs and stops it as well, but the ball just continues straight through cutting of the leg (foot disappears). As the friend falls over screaming a couple of mysterious men runs over, wraps the stump in the plastic-like material, and grabs the friend throwing him into the air where he just speeds of. Then the men runs away just as quickly as they came. Again it’s all over in a couple of seconds.