Scotland Yard is investigating after several posters declared a "fatwa" against Home Secretary Theresa May.
The wanted-style posters in Tooting, south-west London, declare that the fatwa was issued against Mrs May "for the abduction, kidnapping and false imprisonment" of several radical clerics, including jailed hate preacher Abu Hamza.
A fatwa, a religious ruling in Islamic law, can be, but is not always, interpreted as an incitement to kill.
A Scotland Yard spokesman said: "Police are aware of a number of posters in the Tooting area.
"We are liaising with the local authority to get them removed. We are also working to find out who put them up."
Tooting MP Sadiq Khan said the posters were "offensive, divisive and unlawful".
He told the Wandsworth Guardian: "Whoever is behind it should not only face the full extent of the law, but should also foot the bill for damage caused to public property and the cost of taking down these inflammatory posters.
"Tooting is a vibrant, diverse and welcoming place and I take very seriously any attempts by anyone who tries to destabilise our community."
Wandsworth Council said it is working to remove the posters.
Hamza, 52, was jailed for seven years in February 2006 for inciting murder and race hate. He is in Belmarsh Prison as he challenges attempts to extradite him to the US on terror charges.