They said there was evidence the gangs were targeting vulnerable young men and threatening them with violence if they did not "convert" and join them.
Scotland Yard said they were aware of a gang called the Muslim Boys, a number of whom had been arrested for gun and drug offences.
Police and Muslim leaders said the gang's beliefs were not legitimate.
Young people are receptive to this message because they themselves are empty, they need something to believe in
Wanoo Sheshmi, of the From Boyhood to Manhood Foundation, which works with young men in south London, said the gangs tried to get those from the most deprived areas to join them.
He said: "We know that they (the gangs) are targeting young people, vulnerable young people who are receptive to this message because they themselves are empty, they need something to believe in."
Abdul Haq Ad-eye, of the Brixton Mosque, said: "People have been saying that there have been some Muslim youth that have an extremist understanding of Islam, so we are very concerned about that at Brixton Mosque because we have been quite pivotal in warning against extremist understandings of Islam."
Det Ch Supt John Coles, head of Operation Trident, which deals with gun crime within the black community, said most of the "hardcore" of the gang had been arrested.
A police statement said: "Trident are aware of a number of young black males in South London who call themselves the Muslim Boys, and are involved in gun and drug related offences.
"Some of these individuals claim to have converted to Islam, although their beliefs are not recognised by genuine Muslims within the community."
.jpg){kind=small}
.jpg){kind=small}
