London (CNN) -- One moment the teenage girl is walking down a street in London. The next, she lies unconscious on the sidewalk, knocked out cold by a single punch to the head from an unknown assailant.
Shocking closed circuit TV footage released by London's Metropolitan Police shows the attacker jogging up behind the girl as she walks near her home around lunchtime.
He then winds back his right arm, launches a punch that slams her to the ground, and jogs seemingly calmly back down the street in Plaistow, east London, leaving his 16-year-old victim motionless behind him.
Police are appealing for anyone with information on the "unprovoked attack" on November 13 to get in touch.
The man is described as a black man of muscular build, with a shaven head. He is about 6 feet tall, aged 25 to 30 and was wearing a baseball jacket and jeans, according to the Met Police statement.
He had followed the girl for about 500 yards from her home before jogging closer to hit her, police said.
The girl was taken to the hospital for treatment for bruising to the head and cuts to her face. She was discharged later the same day, the statement said.