Around 70 firefighters are tackling a blaze at a factory today.
The fire at the premises in Peterboat Close, Greenwich, south-east London, was being battled by 12 crews, London Fire Brigade said.
They were called to the scene, off the approach to the Blackwall Tunnel, shortly before 3.30am.
The fire brigade said it was still trying to find out what was in the warehouse which was on fire, describing the 80m-long building as "well alight".
Many people reported on Twitter hearing what they described as an "air-raid siren" going off that could be heard across the area.
A man who called himself Dave told BBC London that there was thick smoke on the Old Kent Road, a couple of miles to the west of the fire.
"You can't see your hand in front of your face," he said.
"It is like a smelly fog."
The fire brigade later said the source of the blaze was a chemical processing plant, but that the smoke was not thought to be harmful.
It said that 60 firefighters remained at the scene and due to the heavy smoke it had received a number of calls from people across south-east London.
Assistant commissioner Dany Cotton said: "Our firefighters are working extremely hard to contain the fire, which has now damaged around half of a big industrial unit.
"Heavy smoke has spread throughout the area and as a precaution, we want people to keep their doors and windows closed."
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