Australians in London have spoken of their terror following a third night of riots in the British capital.

Matt Fernon, 22, was managing a pub in southeast London when crowds began rioting.

"It was pretty scary. There were riot police, there were people trying to break into our cars," he told Seven Network News on Tuesday.

"There were people burning things, throwing things on the streets, breaking windows."

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Another Australian in Hackney, Damian Cavanagh, said police were being careful to avoid clashing with rioters.

"At one stage the front line was 15 or 20 officers (who) backed away from one individual who was advancing on them," the 30-year-old said.

"I think they are aware how quickly actions by them could inflame the situation."

Australian journalist James Scurry said parts of the city were "like a war zone".

"There were cars reversing along a major highway. They were trying to escape the violence," he said.

"These types of things just don't happen here in London."

The Australian Department of Foreign Affairs and Trade is warning the more than 50,000 Australians in London to avoid all demonstrations and protests.

-AAP