Worshipers at the Stockwell Green mosque have reportedly received leaflets demanding the killing of heretics.
The group does not inflict violence itself, but it reportedly provides the environment for other people to.
The leaflets found in the building are said to have labelled Ahmadi Muslims apostates, claiming they deserved to die.
One allegedly said: “All possible efforts should be made to clarify the doubts of someone who abandons Islam, otherwise he should be given the punishment of capital punishment.”
It comes as tensions involving the Ahmadis in Britain grow after a Glasgow shopkeeper was allegedly stabbed to death by a Muslim.