Scores of firefighters have spent several hours containing a large fire at a chemical processing plant in Greenwich, southeast London
The fire, in Peterboat Close near the approach to the Blackwall Tunnel, started in the early hours of the morning.
During the height of the blaze, which swept through an 80m-long building, the London Fire Brigade advised local people to keep their doors and windows closed because of heavy smoke.
At times, the smoke was so thick that one eyewitness in the Old Kent Road, about two miles away from the fire, said it was like being engulfed by "smelly fog."
The blaze was brought under control at 6.48am when firefighters said there was no longer any need to keep doors and windows closed. The blaze was not put out completely until about 10.30am.
Many people had reported on Twitter hearing what they described as an "air-raid siren" going off that could be heard across the area from 3.30am.
Assistant commissioner Dany Cotton said firefighters worked "extremely hard to contain the fire, which has now damaged around half of a big industrial unit.
There are no reports of injuries and the smoke was not thought to be harmful.
An investigation is under way to work out how the fire stated.