The most severely affected area was the town of Subic, about 80 kilometres northwest of Manila. Heavy rains there caused rain-soaked soil to cascade down, killing six people, said town mayor Jay Khonghun.
"We've been experiencing very heavy rain. We are now isolated. I can confirm there are two landslide incidents here. Six people were so far killed," Khonghun told AFP.
"The flood water is chest-deep in many areas, and the rain is pounding and the water keeps on rising," he said.
Government employee Cristina Humbert, 35, said the ground floor of her two-storey home had been flooded but she managed to evacuate her 63-year-old mother to higher ground.
More From This Section
"Many are on the rooftops, waiting for help. They are marooned, and are getting hungry and cold. We have no power, no electricity," she told AFP.
Khonghun said four rivers that pass through Subic were heavily silted and had overflowed, causing the floods.
He called on the national government to come to the town's aid.
Local authorities sent rubber boats and fire trucks to pick up residents but there were not enough to cope, Khonghun added.
Classes were called off throughout the Philippine capital and surrounding areas due to flooding that reached more than five feet (1.5 metres) in some places, said Myrna Puzon, desk officer of the National Disaster Risk Reduction and Management Council.
"It is reaching the second floor of houses. Some people have taken refuge on their rooftops," Puzon said.
People there carried their shivering children as they waded through the floodwaters amid submerged cars, an AFP reporter at the scene said.
Typhoon Usagi passed the northern Philippines this weekend and has moved away from the country but it continues to exacerbate monsoon rains.