Danielle Monaghan, who was at the zoo with her two children, partner and his young nieces, said she was worried about safety issues.
The chimps used a branch as a makeshift ladder to escape. But they decided to return. Photo / Twitter, BBC
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."