Flyers encouraging people not to hand over information to police have been posted on an estate where a teenager was shot dead.
The leaflets advertise a website - Stop Snitching - which urges potential informants to refuse to co-operate with the authorities.
It also urged people not to trust Trident, which investigates gun crime in London and has particular expertise in black gunmen and victims.
The leaflets have been left around the Pelican Estate, in Peckham, south east London, where 17-year-old Sylvester Akapalara was shot dead last month.
Investigators offered potential witnesses protection and anonymity if they came forward in the hours after the attack.
Sylvester died after being chased into a stairwell in Heron House. Three teenagers have since been charged with murder.
The leaflets highlight the difficulties police often face in winning the trust of potential witnesses of violent crimes linked to gangs.
It echoes a similar notorious campaign launched in Baltimore, in the United States, in 2004 called Stop Snitchin'.
The phrase was adopted by several crime gangs and hip-hop groups and sparked a police response dubbed Keep Talkin'.
Detective Chief Superintendent Stuart Cundy, who leads the Trident unit, said: "Disappointingly, we are aware of a small amount of irresponsible leafleting in Southwark.
"Trident receives an incredible amount of vital community support across London. Engaging with us is not snitching, but about protecting your community.
"Information from communities is vital to solving murder investigations and we will continue to successfully protect our vulnerable witnesses.
"Anyone with information that could be relevant to police can contact Crimestoppers anonymously on 0800 555 111."
The Stop Snitching website, registered via a hosting company in Belize, claims to have video and audio recordings of police officers arranging protection deals.