Haiyan smashed the into central island the 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 yesterday 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 minimise 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."
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.
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 today.
There were no immediate reports of casualties.
State weather forecaster Glaize Escullar said yesterday 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.
Haiyan had maximum sustained winds today morning of 315 kilometres an hour, and gusts of 379 kilometres an hour, according to the US Navy's Joint Typhoon Warning Centre.