KATHMANDU, Nepal — A powerful earthquake struck Nepal Saturday, killing at least 718 people across a swath of four countries as the violently shaking earth collapsed houses, leveled centuries-old temples and triggered avalanches on Mount Everest.

It was the worst tremor to hit the poor South Asian nation in over 80 years.

It was a few minutes before noon when the quake, with a preliminary magnitude of 7.8, began to rumble across the densely populated Kathmandu Valley, rippling through the capital Kathmandu and spreading in all directions — north toward the Himalayas and Tibet, south to the Indo-Gangetic plains, east toward the Brahmaputra delta of Bangladesh and west toward the historical city of Lahore in Pakistan.

A magnitude-6.6 aftershock hit about an hour later, and smaller aftershocks continued to jolt the region. Within hours, hospitals began to fill up. Kathmandu’s international airport was shut down.

While the extent of the damage and the scale of the disaster are yet to be ascertained, the quake will likely put a huge strain on the resources of this poor country best known for Everest, the highest mountain in the world, and its rich Hindu culture.

The economy of Nepal, a nation of 27.8 million people, is heavily dependent on tourism, principally trekking and Himalayan mountain climbing.

A senior mountaineering guide, Ang Tshering, said an avalanche swept the face of Mount Everest after the earthquake, and government officials said at least 30 people were injured.