Four "violent videos" posted on YouTube have not been removed despite requests from police, the Mayor of London says.
Sadiq Khan said the online firm had refused to take down the knife videos that were first reported in December.
Mr Khan said the videos, showing recognisable London locations, "were a shocking example of the glamorisation of gang culture".
YouTube said it could not comment on individual cases but said it was in contact with the police on this issue.
The videos, which have been viewed more than 356,000 times, shows gangs threatening rivals with a "Rambo knife" while others make shooting hand signals to a soundtrack of violent rap.
A YouTube spokesperson said the site prohibits videos "that are abusive or that promote violence."