As Metropolitan Police officers started to evacuate people from nearby houses, the early indication is that the incident is not terror related.
The alarm was raised around 1:30 p.m. local time after an explosion at premises in Farnham Road, Harold Hill. Local police in Havering urged pedestrians and motorists to avoid the area.
Local media reported that a large part of Romford has been closed off by emergency services after the explosion in a building along a parade of shops in Romford. The report added that Hilldene Avenue area of Harold Hill is on lockdown with a large police cordon in place.
As well as a large police presence, London Fire Brigade and the capital's ambulance service were also called to the scene.
The Met said it was too early to say whether there were any casualties.
"At this early stage officers are unable to confirm the cause of the explosion but enquires are underway," said a spokesman at the Met.
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