Chimpanzees used branches felled by a storm to escape from their zoo enclosure, in front of witnesses, who filmed the whole episode.
This was the second escape in as many months from Belfast zoo, after a red panda escaped in January and was found in a garden less than 1.5km away.
Visitors said they saw the escaped chimps emerge from the bushes on to a path, as others sat on top of their enclosure wall.
Zookeeper Alyn Cairns blamed the recent storm for the escape, but told the BBC that the chimpanzees did not enjoy their taste of freedom.
He said: "They're intelligent primates and know they're not supposed to be out of their enclosure, so got back in themselves."
Danielle Monaghan, who was at the zoo with her two children, partner and his young nieces, said she was worried about safety issues.
She said: "I was petrified, obviously, having the kids, and I tried not to show fear but inside I was a bit like: what happens if it attacks us or tries to take the kids or runs over?"
Elaine Monaghan's grandchildren were at the zoo at the time.
She told ITV: "Why weren't the enclosures checked? This could have ended very badly."
A Belfast Zoo spokesperson said "This is a highly unusual event, despite the natural curious nature of chimpanzees, and we would like to reassure the public that Belfast Zoo is fully committed to providing a safe and enjoyable experience for all our visitors.
"Additional security checks are also being carried out at other enclosures following this incident, to ensure the health and safety of both animals and visitors, which remains a key priority for our dedicated team."