A 20-year-old woman is believed to have fallen and has been airlifted to hospital.
A man whose son found a handbag raised the alarm.
"My son and his friends were up jogging round the mountain and found a handbag and identification," he said.
"They started to look around and they saw a lady lying down the cliff.
"They climbed down the hill and found she was still alive. The alarm was then raised," he told the BBC.
The emergency services are involved in a rescue operation at Cavehill Country Park in north Belfast.