Four teenagers have been stabbed in the Streatham and Brixton area in the past fortnight in so-called "tit-for-tat attacks".
Detective Inspector Andy McDonald, from Streatham police, said officers believe the incidents are revenge attacks from rival gang members.
He said they seem to be intended to humiliate the victims, rather than to seriously injury them, with some of the youths being stabbed in the leg. "But if they slip we would have something more serious on our hands," he said.
"Despite extensive media campaigns it seems that certain young people are continuing to carry and use knives. These people must understand police will arrest and charge them for doing this. We are taking this very seriously."
One youth has been charged and other arrests have been made in relation to the attacks.
Anyone with information should call 020 8649 2936 or Crimestoppers anonymously on 0800 555 111.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article