The animals are known to be strong and intelligent
Thirty chimpanzees have escaped from their enclosure at a zoo in Cheshire, forcing it to be closed.
The animals found their way into a nearby keepers' area, where their food is usually prepared, at about 1240 BST, Chester Zoo said.
A spokeswoman for the zoo said the animals had been contained and the zoo was closed as a security measure.
How the animals came to leave their quarters is not known but they were not in an area accessed by the public.
Sarah Jones, from Chester, was among the visitors at the zoo when the animals left their enclosure.
"My two young boys had only just gone into the chimpanzee house when we heard terrifying loud noises," she said.
"The chimpanzees were large animals and they seemed very upset about something, they were making a frightening noise, I thought something was wrong, so we made our way out of the building.
"I saw keepers running into the house and visitors were asked to head to other areas of the zoo.
"We ended up being stuck in the cafe area for quite a while."
Chimpanzees are considered to be the most intelligent non-human primate. They are mainly terrestrial, but do spend time in the trees.
A zoo spokeswoman described them as very strong animals but said there had been no injuries to any member of the public or to staff.
"The decision to evacuate the zoo was purely taken as a precautionary measure," the zoo said in a statement.
"We apologise for any inconvenience this has caused to our visitors and would thank them for their full co-operation and patience."