London:  A judge blasted Britain's "dreadful" gang culture as he jailed a group of youths on Friday  who had stabbed a 16-year-old boy to death in a London street.

Kodjo Yenga was ambushed by about a dozen teenagers, including two girls, armed with knives and bats and yelling "kill him". After attacking the student in front of horrified passers-by, the gang ran off smiling and laughing with their bull-terrier dog, prosecutors said.

He lay dying in his girlfriend's arms, after being stabbed in the heart during the attack in Hammersmith, west London in March last year. Passers-by, including a policeman, tried to help him after he collapsed in the street but he later died in hospital.

Yesterday the gang, from Hammersmith and Shepherds Bush, were jailed despite denying their involvement. Passing sentence at the Old Bailey, judge Christopher Moss said it was "another needless loss of a young life on the streets of our cities".

"You were all part of gang culture which casts its dreadful influence over our cities and leads to the sort of tragedy we have seen here," he told the court. "The courts must send out a clear message that the violent taking of lives by youngsters... must be punished severely."