Chester Zoo has tweeted that its teams are working to move all animals away from the blaze, which firefighters are working to extinguish.
Every effort has been made to minimise disruption in the local area and I would also like to thank members of the public for their co-operation and understanding.
"The zoo's The Lanterns event this evening has been cancelled and ticket holders notified by email".
Visitors and animals are being evacuated from the popular attraction in Cheshire, which is home to more than 21,000 animals.
A number of shocking videos and photos were shared on social media, showing large plumes of smoke coming from the zoo.
The zoo has closed and we have no indication of when it will reopen.
"Thank you for the overwhelming messages of goodwill following the fire in our Monsoon Forest building; they have been of great comfort to our teams".
PETER BYRNE APThe zoo which did not give a cause for the fire said it worked with emergency services to bring the situation under control
Some zoo visitors tweeted images from the scene including Sophie Flynn, who said staff were "working hard to deal" with the fire, which "happened very quickly". Many staff (were) running towards the fire, (I'm) assuming to help with evacuating animals. Emergency services were called at 11:40am, and no humans are believed to have been injured.
"Crews - wearing breathing apparatus and using main jets - are now tackling the blaze in a building on the site, which is 100m by 25m".
Thankfully, it's since been announced that all animals are safe and there are no injuries reported.
"Drivers are advised to avoid the area whilst emergency services continue to work at the scene".
"The fire was reported at 11.28am and there are now 15 fire appliances in attendance at the scene".
Due to the scale of the fire this was declared a major incident, a statement says.
"We will provide further updates when we can".