Martin was stabbed in the chest
A 14-year-old boy was stabbed to death after two groups of teenagers pulled faces at each other, a court has heard.
Martin Dinnegan was knifed in the chest during a dispute involving up to 20 youths in Islington in June 2007.
The row began when a group on a bus and another group on bikes pulled faces at each other, the Old Bailey heard.
Rene John-Baptiste, 21, of Plaistow, east London; Sean Clarke, 19, of Holloway, and two teenagers aged 16 and 17, deny murder.
Aftab Jafferjee QC, prosecuting, said teenagers posturing in front of their friends and being prepared for violent confrontations was now a scourge of towns and cities.
"This is such a case, which leads from dirty looks to death in one hour," he said.
'Scourge of posturing'
Martin was on the bus with friends. Mr Jafferjee said the 16-year-old accused of killing Martin was part of the group on bicycles.
"The growing scourge of urban posturing involves aimless meanderings by groups of young people who are prepared to exploit any pretext to engage in, or threaten to use, violent behaviour," he said.
"An essential part of such posturing is the importance in the minds of these teenagers of not just being prepared to use weapons but to be seen to be doing so by their peers."
After the groups exchanged "dirty looks" the youths on cycles followed the group on the bus.
The groups clashed twice - the first time in a nearby park and one-and-a-half hours later close to a housing estate.
The jury heard both groups had a knife each and a screwdriver during the fight.
Martin fled from the final confrontation "running for his life" but he was chased by the defendants who gave chase on cycles and mopeds and "a mob attack was unleashed" resulting in his death, Mr Jafferjee said.
The trial has been adjourned until Friday.