Severe Tropical Cyclone Gita has already created havoc in neighbouring Samoa and is gathering pace as it approaches Tonga.
Acting Prime Minister Semisi Sika issued a nationwide alert, saying he was "satisfied that an emergency is happening or is about to happen".
The Fiji Meteorological Service predicted Gita will become a Category Five storm -- the top of the scale -- before reaching Tonga Monday night.
It is already packing gusts of 275 kmh as it sits off the east coast of the country's most populous island Tongatapu.
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Gita slammed into Samoa overnight Friday, forcing the evacuation of some 200 people and causing widespread flooding, leaving many without power.
Philip Duncan, chief forecaster at New Zealand's Weather Watch service, said that current modelling showed Gita was lining up a direct hit on Tongatapu.
"That's a very serious situation, the capital (Nuku'alofa) is there, there's over 75,000 people," he said.
"It's pretty rare to see the perfect circle, the centre of that storm, going right over the top of such a small island."
Duncan said the cyclone could cause major damage even if it remained offshore.
Truckloads of troops were out helping people batten down as Nuku'alofa prepared for the cyclone and evacuation centres were opened across the kingdom.
The information ministry advised anyone considering moving to the shelters to leave early, saying travel would not be possible when the eye of the storm passes over.
Otherwise people should stay at home and ride out the wild weather, it said.
"Everyone should be mindful of what might cause damage within their homes such as large trees that can potentially damage a house."
Tonga Meteorological Service director Ofa Fa'anunu said Gita would be one of the strongest cyclones to ever hit the kingdom.
"We are looking at major structural damage," he warned, pointing out that the low-lying capital was particularly vulnerable to storm surges.
Cyclones are common in the Pacific at this time of year, with top-of-the-scale Category Five systems proving highly destructive when they make landfall.