An east London mosque was put on lockdown after a man fired a gun after being forced to leave.
Armed police were scrambled to the Seven Kings Mosque in Ilford on Thursday night after a "masked" gunman entered the building during Ramadan prayers.
The man reportedly entered the mosque but was made to leave before firing the gun outside.
Scotland Yard said early investigations suggested a "blank firing handgun" had been discharged.
The force said no injuries or damage to the building were reported and detectives were not treating the incident as terrorism-related.
The main entrance of an east London mosque remained closed this afternoon as police investigate the incident.
A side door of the Seven Kings Mosque in Ilford was reopened for worshippers to use but an officer at the scene said it was unlikely the mosque would reopen fully until Friday night or Saturday morning.
Worshipper Ibraheem Hussain, 19, described hearing the gunshot around half an hour after evening prayers began.
He told the Press Association he heard a loud noise that sounded “like a firework or maybe something heavy had been dropped”, and continued his prayers.
"After we finished we were leaving the shoe area and someone said it was a gunshot, someone had come into the mosque and he had a firearm on him,” he said.
"The managers had seen him, he was masked and acting suspicious and the firearm was let off."
The mosque’s imam Mufti Suhail issued a statement saying: “There was a firearm incident outside Seven Kings Mosque tonight during Ramadan night prayers.
"The suspect ran from the scene when stopped by brothers standing guard at the mosque.
"A shot was fired which fortunately did not hit anyone."
Scotland Yard said police were called at 10.45pm on Thursday to reports that a man, believed to be in possession of a firearm, had entered the mosque
"The man was ushered outside of the building by those inside. A gunshot was then heard,” the force 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 continue to work closely with representatives from the mosque and are providing reassurance to the local community."
The incident comes amid heightened concerns over security at places of worship around the world following recent attacks in New Zealand, Sri Lanka and the US.