Koppu, known as Lando in the Philippines, had sustained winds of 240 kph (149 mph), a strength equivalent to a high-end Category 4 hurricane.
Koppu's eye made landfall at about 1 a.m. local time over Casiguran, a coastal city of about 25,000 people on Luzon, the country's largest and most populous island.
Even before the eye hit, the storm's outer bands lashed the country's east coast with heavy rain. Casiguran had 105 mm (4.1 inches) by late Saturday night, while Virac, on an island to the south, had 166 mm (6.5 inches).
The country's PAGASA weather service warned of likely significant damage to buildings and trees near the eye, and of dangerous storm surges that could rise as high as 3 meters (9.8 feet) along the coast.
But the danger for eastern and northern Luzon could last for three days, because Koppu is expected to go up the island at an excruciatingly slow pace.
Some areas could get more than 19.6 inches of rain by early Tuesday. By the end of Wednesday, as much of 1 meter (39 inches) of rain could have fallen in some places, CNN meteorologist Allison Chinchar said.
"That's where the problem with the flash flooding comes in, because when you have all of this rain that keeps coming down over the same places over and over, that is likely to trigger mudslides and landslides in addition to flash flooding problems in ... some of the low-lying areas," Chinchar said.
The Philippines is frequently hit by typhoons. In December, for instance, Hagupit killed at least 18 people and injured hundreds more.
But that devastation paled in comparison to the havoc wrought by Super Typhoon Haiyan in November 2013, which killed more than 6,000 people and injured more than 27,000 others. That typhoon, considered to be among the strongest storms ever to make landfall, hit the eastern city of Tacloban, well south of Luzon, especially hard.