Megi, the 10th and strongest typhoon to hit the Philippines this year, reached Isabela province on Monday morning and by early evening was heading west-south-west across the north of the main island of Luzon with winds of 180 kilometres per hour (kph) near the centre, forecasters said.
The Red Cross says Megi is the biggest typhoon to hit the Phillippines in decades, and that its impact could be devastating.
"It's a very, very big typhoon that's hitting our country, biggest in the last 20 years," Philippine National Red Cross chairman Senator Richard Gordon said.
"It could inundate coastal areas, river banks and it might cause landslides.
Tropical Storm Risk said Megi, known locally as Juan, was a category 5 super typhoon, the highest rating, with winds of more than 250 kph when it hit mountains in north-east Luzon.
"The governor of Isabela declared a state of calamity, so there could be massive damage and destruction there," Benito Ramos, executive director of the national disaster agency, said.
"Power has been cut and crops about to be harvested could have been destroyed. We have no actual report because we're waiting for the weather to clear up to make an assessment."
Initial reports were of one death and a small number of casualties, although the National Telecommunications Commission said up to 90 per cent of communications in Isabela and Cagayan provinces may have been knocked out.
Television footage showed uprooted trees on roads and metal and thatched roofing blown off houses.