A gun was fired outside a mosque in east London during Ramadan prayers.
Police were called to reports of a man with a firearm entering the Seven Kings Masjid in Ilford on Thursday night.
Worshippers ushered him out of the building and a gunshot – thought to have come from a “blank-firing handgun” – was then heard.
No injuries or damage were caused, Scotland Yard said, and it did not believe the incident was terrorism-related.
Evening prayers were being held at the mosque, off High Road, at the time.
The Met Police said: “Officers, including firearms officers, attended. There were no reported injuries or damage to the building.
“At this early stage, ballistic evidence recovered from the scene suggests that the weapon was a blank-firing handgun.”
Officers will remain at the scene overnight, it added.
There are heightened security concerns at places of worship around the world after recent attacks.
A mass shooting at a mosque in Christchurch, New Zealand, in March, left 50 people dead.
In April, churches in Sri Lanka were targeted on Easter Sunday in a terror attack which killed at least 253 people.
A week later, a woman was killed when a gunman opened fire at a synagogue in California.