Thousands of residents in the city of Iloilo were left stranded on rooftops after a dam burst.
"I have received a lot of text messages appealing for helicopters, there are many people trapped on the rooftops," Serg Biron, an Iloilo congressman, told DZBB radio.
"This is the worst flooding that has hit Iloilo in history."
The National Power Corporation was forced to shut down its power plant in the area, triggering a blackout across the province, Nelson Hemona, the plant's manager, said.
Fengshen, the sixth typhoon to hit the Philippines this year, tore up trees and power lines across wide swathes of the country. Flights were cancelled and ships kept in harbour.
The storm forced more than 200,000 people to seek temporary shelter in the eastern Bicol region, a civil defence office said.
Gloria Arroyo, the country's president, ordered clean-up and rescue operations before leaving for an eight-day trip to the United States.
Government agencies were instructed to stockpile relief goods and state-run hospitals were put on alert.
Arroyo also ordered a crackdown on profiteers and hoarders of basic commodities, especially rice, in areas hit hard by the typhoon.