Firefighters issued a warning for south-east Londonders to keep their "doors and windows closed" after a blaze at a chemical processing plant in Greenwich on Monday.
The fire at the premises in Peterboat Close, Greenwich, south-east London, was battled by 12 crews overnight, London Fire Brigade said.
They were called to the scene, off the approach to the Blackwall Tunnel, shortly before 3.30am.
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."
The fire brigade later said the blaze was out, tweeting.
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."
WATCH: Video of Greenwich fire