The National Parks Board said that the incident took place at about 5.30pm, when the northern part of Singapore was experiencing thunderstorms and gusty winds.
At the time, a group of 17 people were having makan at the pavilion to celebrate Christmas.
It started to rain heavily and the tree suddenly uprooted and collapsed.
12 people in the group were trapped under the pavilion while another 5 escaped unhurt.
SCDF officers managed to extricate the trapped persons and they were sent to hospital for treatment.
The tree, a Erythrophleum Suaveolens, was one of 2 trees at the park that collapsed.
NParks is investigating the cause of the incident.