A cafe, including an animal petting area, and gift shop are said to be "currently alight".
A spokesperson for the zoo confirmed animals were in the vicinity but did not comment on their safety.
The fire broke out near the old penguin pool towards the back of the site.
London Fire Brigade said in a statement: "Ten fire engines and 72 firefighters and officers have been called to a fire at a cafe and shop at London Zoo.
"Approximately half of an adventure cafe and shop, which also contains an animal petting area, is currently alight.
"It is not known if any animals are involved at this stage.
"The Brigade was called at 0608. Fire crews from Kentish Town, Paddington, Euston, West Hampstead and other surrounding fire stations are at the scene.
"The cause of the fire is not known at this stage."
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