Typhoon Haiyan is battering the central 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.
The storm has forced millions to seek shelter in 20 provinces and at least two people have died, officials say.
The region was already struggling to recover from a powerful earthquake last month.
The authorities have warned that more than 12 million people are at risk from the category-five storm, including in Cebu, the country's second largest city with a population of 2.5 million.
The storm - known locally as Yolanda - was 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."
Schools and offices closed, while ferry services and local flights were suspended. Hospitals and soldiers are on stand-by for rescue and relief operations.
The extent of the damage remains unclear, with authorities saying phone-lines are down in many areas.
The governor of the Southern Leyte province, Roger Mercado, tweetedon Friday morning that fallen trees were blocking roads, hampering the emergency effort.
Roxane Sombise, a resident of Tacloban, in Leyte, told the BBC: "I think our house is actually shaking... I just want it to stop."
Protestant pastor Diosdado Casera in Surigao City in north-east Mindanao said the city had missed the worst of the storm but there was a lot of damage.
"The storm was very strong - although Surigao City was not directly hit we experienced its fury early this morning," he said.
"The big buildings made of concrete were fine, but the houses made of wood and shingles and plywood have suffered a lot of damage, mainly to their roof."
A spokesperson for the British Red Cross, Nichola Jones, who is in Tagbilaran in Bohol, says the typhoon passed by in the past hour.
"We've lost power and a few roof-tiles but it's not too bad. But I think to the North - that's the area that has borne the brunt. Those were the areas worst hit by the earthquake last month."
In the worst-hit areas of Samar and Leyte, she says there are reports of collapsed buildings, including a hotel.
"In Cebu they have had quite a battering and I spoke to our colleagues and they've had quite strong winds and are locked down in their hotels. They are waiting to see what the situation is."
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 Typhoon Haiyan had made landfall over Guiuan at 04:40.
The typhoon 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).
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.
Earlier, President Benigno Aquino warned people to leave storm-prone areas and urged seafarers to stay in port.
"No typhoon can bring Filipinos to their knees if we'll be united," he said in a televised address.
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.
It is the 25th typhoon to enter Philippine territory this year. (BBC)