The intelligent primates turned a tree into a makeshift ladder and scaled high walls surrounding their pen in Northern Ireland.
Visitor Danielle Monaghan, who shared video of the daring escape on Facebook, told the BBC she was "petrified" the chimp might "attack or take the kids". She wrote that one of the animals came close to her grandchildren, with "Belfast Zoo trying to make out it wasn't a big deal". However, the animal did not seem threatening, to Ms Monaghan's relief.
In November previous year, a snow leopard was shot dead after the animal escaped from its enclosure in Dudley Zoo in Britain.
She said she and her companions "could not believe it" when the "smart" chimpanzees started to climb onto the branch and hopped over the wall.More news: Leeds teenager Clarke taken to hospital after touchline treatment
She said she was "scared" but called it an "amazing" experience and said the chimp appeared to be "very calm". "We just walked past it and it was absolutely grand".
Visitors to Belfast Zoo were left "petrified" after a chimpanzee escaped from their enclosure, using a branch as an improvised ladder.
"They're intelligent primates and know they're not supposed to be out of their enclosure, so got back in themselves", Mr Cairns told the BBC. The great escape ended when the animals returned to their quarters.
He said the zoo would conduct a safety review and possibly remove some of the trees, whose limbs might facilitate future breakouts.More news: Judge Approves Lampert's Sears Purchase
Last month, an endangered red panda cub escaped from Belfast Zoo.It was eventually found in a residential garden a short distance from the zoo.
Alan Cairns, from the zoo, added that chimpanzees were locked into their inner enclosure following the break-out.
A zoo spokesperson told media that the chimpanzees were only briefly out of their enclosures as zoo personnel were on hand to return them.
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