The category 4 cyclone is expected to hit Tongatapu and Eua around 7pm today.
The government has declared a State of Emergency until 12 March 2018.
All the schools were closed today and some public and private agencies suspended their services as part of their preparations for the cyclone.
Evacuation centres in villages on Tongatapu are now open and the public is being urged to move persons with disabilities and other vulnerable groups to the centres while there is still daylight.
Residents were also securing their properties by tying down roofs and covering windows with corrugated iron sheets.
They have also been stocking up on food, water and fuel supplies.
The Director of Tonga’s Meteorology Office, 'Ofa Fa'anunu told a local radio programme this afternoon that the cyclone is expected to intensify to category 5 by this evening.
The eye of the cyclone is close to Tongatapu and Eua islands.