Thousands of people were being evacuated from their homes in the Philippines on Friday, as Super Typhoon Mangkhut, a colossal storm more than 550 miles wide with maximum sustained wind speeds of 173 miles per hour, howled its way across the Pacific.

Mangkhut’s eye is on course to hit in the early hours Saturday on the northern island of Luzon, the country’s rice and corn growing heartland, where more than four million people are at risk.

The storm, gusting at speeds equivalent to a Category 5 hurricane, passed the American territory of Guam on Thursday, knocking out 80 percent of the island’s electricity and downing trees and power lines.

Across the Philippines, schools have been shuttered, home and business owners have boarded their windows and the military has been put on high alert.

President Rodrigo Duterte barred troops from taking leave, and ordered that rice seized by customs officials at the country’s ports should be turned over to the Department of Social Welfare and Development for potential disaster relief.

Hundreds of bulldozers were made ready in the event of landslides, and rescue workers were being deployed across the country. In some cases, Mr. Duterte said, resources that had already been dispatched were being moved to get them out of the path of the storm.

President Duterte warned that the storm could deal a severe blow to the country’s agricultural sector, just as the rice and corn harvests are set to start.

The president’s order that farmers harvest their most mature grains immediately set up a difficult choice for farmers who were also told to evacuate.

If the country was hit hard by the storm, the president predicted hundreds of millions of dollars in lost revenue.

After the Philippines, the storm is predicted to pass Hong Kong on Sunday before slamming into the Chinese mainland on Monday morning.

The Hong Kong Observatory warned residents of the territory to “take suitable precautions and pay close attention to the latest information” on the storm.

In mainland China, the southern provinces of Guangdong, Guangxi and Hainan have ordered residents to seek shelter away from the coast.