The typhoon named Super Typhoon Haiyan smashed into the central island of Samar, about 600 kilometres southeast of Manila, at 4.40am (2040 GMT Thursday) and was travelling quickly northwest, state meteorologist Romeo Cajulis told AFP.
President Benigno Aquino had on Thursday warned his countrymen to make all possible preparations for Haiyan, which was packing monster wind gusts of nearly 380 kilometres (235 miles) an hour as it approached the Philippines.
"To our local officials, your constituents are facing a serious peril. Let us do all we can while (Haiyan) has not yet hit land," Aquino said in a nationally televised address.
"We can minimize the effects of this typhoon if we help each other. Let us remain calm, especially in buying our primary needs, and in moving to safer places."
Haiyan had maximum sustained winds as it approached the Philippines on Friday morning of 315 kilometres an hour, and gusts of 379 kilometres an hour, according to the US navy's Joint Typhoon Warning Centre.
The Philippine state weather service, which typically gives lower wind readings, said the maximum gusts on Friday morning were 275 kilometres an hour.
But even that reading would easily make Haiyan the world's strongest typhoon this year, according to David Michael Padua, a meteorologist with the Weather Philippines Foundation, a storm monitoring organization that runs the www.weather.com.ph website.
A prominent American meteorologist, Dr Jeff Masters, wrote on www.wunderground.com that Haiyan was one of the most intense typhoons ever recorded.
Aquino warned areas within the expected 600-kilometre typhoon front would be exposed to severe flooding as well as devastating winds, while coastal areas may see waves six metres (20 feet) high.
In the eastern coastal city of Tacloban, one of the first areas to be hit, streets were flooded and some buildings were torn down, according to footage broadcast on ABS CBN television on Friday morning.
More than 125,000 people in the most vulnerable areas had been moved to evacuation centres before Haiyan hit, according to the civil defence office, and millions of others braced for the typhoon in their homes.
Authorities said schools in the storm's path were closed, ferry services suspended and fishermen ordered to secure their vessels.
In the capital of Manila, which was not directly in the typhoon's path but still expected to feel some of its impact, many schools were also closed.
Philippine Airlines, Cebu Pacific and other carriers announced the suspension of hundreds of flights, mostly domestic but also some international.
Cajulis said Haiyan was travelling quickly, at 39 kilometres an hour, and would travel across the country towards the South China Sea throughout Friday.
There were no immediate reports of casualties. State weather forecaster Glaize Escullar said on Thursday Haiyan was expected to hit areas still recovering from a devastating storm in 2011 and from a 7.1-magnitude quake last month.
They include the central island of Bohol, the epicentre of the earthquake that killed 222 people, where at least 5,000 survivors are still living in tents while waiting for new homes.
Other vulnerable areas are the port cities of Cagayan de Oro and Iligan on the southern island of Mindanao, where flash floods induced by Tropical Storm Washi killed more than 1,000 people in December 2011.
The Philippines is battered by an average of 20 major storms or typhoons each year, many of them deadly, but scientists have said climate change may be increasing their ferocity and frequency.
The Philippines endured the world's strongest storm of 2012, when Typhoon Bopha left about 2,000 people dead or missing on Mindanao island in December.
(With agency inputs))