London Fire Brigade (LFB) said the fire that spread to most of the Adventure Cafe and shop, which contains an animal petting area, was now under control.
No injuries have been reported and the LFB said it was not yet known if any animals were involved. The cause of the fire is not yet known.
London Zoo tweeted it would be shut on Saturday “until further notice” and would update later.
Ten fire engines were called to the zoo, which sits in the capital’s Regent’s Park, shortly after 06:00 GMT and the fire was brought under control about three hours later.
Station Manager Clive Robinson, who is at the scene, said three-quarters of the cafe and shop had been affected by the fire and half of the roof.
He said: “Part of nearby animal petting area was also affected although at this stage we don’t know if any animals were involved.
“Firefighters worked hard to bring the fire under control as quickly as possible and to stop it from spreading to neighbouring animal enclosures.
“Crews will remain at the scene throughout the morning damping down any remaining pockets of fire.”
A photograph posted on Twitter by LFB shows smoke billowing from the building as firefighters worked at the scene.
London Zoo tweeted it was “currently dealing with an incident on site after a fire broke out”.
It added: “We’d like to reassure you that the fire is now under control. The Zoo will be closed today until further notice. We will update as quickly as the situation allows.”
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