"Police were called at 10.45 p.m. on Thursday, May 9, to reports that a man, believed to be in possession of a firearm, had entered a mosque on High Road in Seven Kings," Met Police told the Daily Star. "The man was ushered outside the building by those inside. A gunshot was then heard."
The mosque is located in London's eastern Ilford district. Police say they'll keep a large presence in the area overnight.
The incident comes during the Muslim holy month of Ramadan, when practicioners are expected to fast from sunup to sundown as an expression of self-discipline and sacrifice.
One person wrote on Twitter they were upstairs performing tarawih, or special prayers associated with Ramadan that Sunni Muslims perform after dark, and "thought it was a firework or something."
Another wrote that it was a pellet gun, and the person was "chased away by the people."
Tensions are high surrounding Muslims places of worship, amid a wave of mass shootings and attacks on mosques in recent months. Mass shootings in Poway, California, and Christchurch, New Zealand, have left dozens of worshipers dead and millions terrorized.