LONDON (AFP) - Six people were treated by medics Monday and one hospitalised after a blaze at industrial units in south London sent thick smoke billowing across the British capital and disrupted transport.

At its height, some 100 firefighters were sent to tackle the fire in three commercial units under railway arches at Elephant and Castle, near the River Thames.

Six cars and a telephone box also caught fire, London Fire Brigade (LFB) said.

London mayor Sadiq Khan said the fire was brought under control, but the ambulance service said it treated six people at the scene including for smoke inhalation, and took one of them to hospital.

"I want to thank our brave emergency services... who acted quickly to ensure both residents and passengers at the nearby station were safely evacuated," Khan said.

LFB said it received dozens of emergency calls from the public and had sent 15 fire engines from across the capital to the scene.

"The cause of the fire is not known at this time," it added. But the Metropolitan Police said it was not believed to be terror related.

Social media footage showed dense smoke risking from the units, and a fiery explosion that caused emergency service personnel to take cover.

Thick, black smoke could be seen from office blocks across the river in central London.

Roads were closed in the area while National Rail warned that trains would be cancelled or delayed until Monday evening, given the proximity of the station to the fire.