HONG KONG — Tonga began cleaning up on Tuesday from the most powerful storm to hit the South Pacific island nation in at least 60 years, which flattened its Parliament building, damaged houses and left thousands without power.

Tropical Cyclone Gita was measured as a Category 4 storm, with sustained winds of up to 145 miles per hour, when it crossed south of Tongatapu, Tonga’s main island, late Monday.

The storm knocked down the kingdom’s century-old Parliament building and tore off roofs in the capital city of Nuku’alofa.

“It was a terrifying night,” Graham Kenna, an adviser to Tonga’s National Emergency Management Office, told the Australian Broadcasting Corporation. “We’re on the third floor of one of the safest buildings in Nuku’alofa and the building was shaking and getting pounded by debris from missing roofs.”