Typhoon Haiyan is battering the Philippines with sustained winds of 235 km/h (146mph).
Meteorologists say that if initial estimates based on satellite images are borne out, it could be the most powerful storm ever to make landfall.
Schools and offices have been closed in the path of the storm, and thousands of people have been evacuated amid fears of serious damage.
The region was already struggling to recover from an earthquake last month.
The category-five storm was centred 62km (40 miles) south-east of Guiuan, in the country’s Eastern Samar province, the national weather service said.
The governor of the Southern Leyte province, Roger Mercado, tweetedon Friday morning that fallen trees were blocking roads, hampering the relief effort.
The storm is not expected to directly hit the capital Manila, further north.
Mai Zamora, from the charity World Vision, in Cebu, told the BBC: “The wind here is whistling. It’s so strong and the heavy downpours are continuing.”
“We’ve been hearing from my colleagues in [the city of] Tacloban that they’ve seen galvanised iron sheets flying just like kites. It’s actually all around the roads now. The roads are flooded in Tacloban,” she added.
Roxane Sombise, a resident of Tacloban, in Leyte, told the BBC: “I think our house is actually shaking… I just want it to stop.”
A teacher in Southern Leyte province told a local radio station that her school was “now packed with evacuees”.
Jeff Masters, meteorology director at the private firm Weather Underground, said in a blog post that the damage from Haiyan’s winds must have been “perhaps the greatest wind damage any city on Earth has endured from a tropical cyclone in the past century”.
State meteorologist Romeo Cajulis told AFP news agency Haiyan had made landfall over Guiuan at 04:40.
The typhoon, known locally as Yolanda, arrived with gusts of up to 275 km/h (170 mph), the Philippines’ weather service said in its bulletin, issued at 05:00 local time (21:00 GMT).
The US Navy’s Joint Typhoon Warning Centre, which typically gives higher readings as they are based on a shorter period of time, said shortly before Haiyan’s landfall that its maximum sustained winds were 314 km/h (195 mph), with gusts up to 379 km/h (235 mph).
Authorities in Guiuan could not immediately be reached for word of any deaths or damage, regional civil defence chief Rey Gozon said.
Forecaster Mario Palafox with the national weather bureau said it had lost contact with its staff in the landfall area.
Waves as high as 5m (15ft) could be seen from the islands of Leyte and Samar, Reuters news agency reported.
The storm is forecast to move over to the South China Sea north of Palawan Island on Saturday, meteorologists say.
In its path are areas already struggling to recover from a 7.3-magnitude earthquake last month, including the worst-hit island of Bohol.
About 5,000 people are still living in tents in Bohol after losing their homes in the quake, which killed more than 200 people.
The military says it is transporting food packages and relief goods to remote communities, and has helicopters on stand-by. Ferry operations are suspended and fishing boats were ordered back to port ahead of the storm.
Thousands of people from villages at risk across several provinces have been evacuated, while schools and offices have shut.
President Benigno Aquino warned people to leave storm-prone areas and urged seafarers to stay in port.
He reassured the public that cargo planes and military helicopters were on standby, along with 20 navy ships.
“No typhoon can bring Filipinos to their knees if we’ll be united,” he said in a televised address.
Ben Evardone, a member of Congress from Eastern Samar, said earlier he had “issued a call to prepare for the worst”.
Meteorologists in the Philippines warned that Haiyan could be as devastating as Typhoon Bopha in 2012.
Bopha devastated parts of the southern Philippines, leaving at least 1,000 people dead and causing more than $1bn (£620m) in damage.
“This is a very dangerous typhoon, local officials know where the vulnerable areas are and have given instructions on evacuations,” state weather forecaster Glaiza Escullar told AFP.
“There are not too many mountains on its path to deflect the force of impact, making it more dangerous.”