Abdul Hadi Arwani, 48, was found in his car with gunshot wounds to the chest in Wembley, northwest London, tomorrow.
The investigation into his death is being handled by counter-terrorism police.
Scotland Yard said a 46-year-old man had been arrested on suspicion of conspiracy to murder and remains in police custody.
Arwani reportedly fled Syria as a teenager after surviving the 1982 Hama massacre, in which the current president's father Hafez al-Assad sent troops to brutally crush an Islamist uprising.
Police say they are "open-minded" about the motive for his killing.
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