Tonga declared a state of emergency Monday and imposed a curfew in the capital Nuku'alofa as the Pacific island kingdom braced for a direct hit from Severe Tropical Cyclone Gita.

The cyclone has already created havoc in neighbouring Samoa and is threatening to become a Category Five superstorm 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 predicts 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 (170 mph) as it sits off the east coast of the country's most populous island Tongatapu.

Truckloads of troops were helping people batten down as the capital prepared for the cyclone and evacuation centres were opened across the kingdom.

Police also announced an overnight curfew in central Nuku'alofa "to further protect people and property".