Hundreds of people were evacuated from their homes in Tonga Saturday when severe tropical cyclone Ula slammed into the tiny Pacific kingdom.

However, there were no casualties after Prime Minister Akilisi Pohiva declared a state of emergency before the storm hit "in order to prevent or minimise the loss of human life, illness or injury".

The northern island of Vava'u took the brunt of the category three cyclone wich was packing winds up to 150 kilometres per hour (93 miles per hour).

"We are very happy that there are no casualties, police checked with hospitals and town officers to confirm that," the chairman of the National Emergency Management Office Siaosi Sovaleni told reporters at a press conference.

"We had 11 evacuation centres, over 390 people were relocated to these evacuation centres."

The storm brought down trees, damaged crops, and the main airport was closed to domestic flights.

The director of the Tonga meteorological office, Ofa Fa'anunu, said it was fortunate there was little moisture around which would have intensified the severity of the storm.

"On another day it would probably have become a (maximum) category five," he said.

"If the moisture had been present it would have been the right ingredients for development into a much stronger system."

The official Tropical Cyclone Warning Centre said in an advisory that Ula had "very destructive winds" with "very high to phenomenal seas" causing flooding in low-lying areas.

The hurricane warning for Vava'u was downgraded to a storm warning early Saturday afternoon.

The tropical cyclone warnings for other island groups in Tonga were also downgraded.

The storm is expected to weaken Sunday as it heads towards Fiji.

A year ago Tonga was hit by Cyclone Ian which left one person dead, 4,000 homeless and destroyed crops on outlying islands.