Stephen Lawrence "ran for his life" after a fatal attack lasting just 10 seconds, a court has heard.
Eyewitness Royston Westbrook told a jury at the Old Bailey that he initially thought the black teenager had only been punched.
He said: "It was so quick, it really was quick - it was about 10 seconds - and my impression going away from that was that they managed to punch him and that was it."
He said Mr Lawrence got back to his feet almost as soon as he was forced to the ground, and added: "He basically crossed over the road and ran for his life up the hill."
He said the four or five white youths who had staged the attack then just walked away.
Mr Lawrence was killed by a gang in Eltham, south-east London, in April 1993.
Gary Dobson, 36, and David Norris, 35, both of south London, deny his murder.
Hospital worker Mr Westbrook said he saw a group of white men "collide" with Mr Lawrence and his friend Duwayne Brooks.
Re-living the attack, which happened more than 18 years ago, Mr Westbrook told the court: "They grabbed Duwayne Brooks' wrist but he turned his wrist and just pulled away. Duwayne ran towards the bus stop where we were standing and turned round and shouted something like 'Leg it, Steve' or 'Run, Stephen'."
Mr Brooks managed to run away but Mr Lawrence was forced to the ground.
Mr Westbrook told the jury: "I saw them surround Stephen and he went down basically through sheer weight of numbers in the middle of them... it looked at the time that someone went to punch him."