At least 19 people have died in severe floods in the Philippine capital, Manila, and nearby areas.
More than 80,000 people are being looked after in emergency shelters, with torrential rain leaving low-lying areas underwater.
Rescuers are using rubber boats to reach stranded people, but some have refused to leave amid fears of looting.
The flooding - neck-deep in some parts of the city - forced the closure of offices and schools.
More than half the amount of rain normally seen in August has fallen in the capital in 24 hours, reports say.
In the worst reported incident of casualties, nine members of one family died when a landslide hit shanty houses in Manila's Quezon City.
Others died from drowning and electrocution, according to the country's disaster response agency. A state of calamity has been issued in at least four areas, it added.
Benito Ramos, head of the country's disaster response agency, said that at least 60% of the city was underwater.
"We're still concerned about the situation in the coastal areas," he added. "It was difficult to distinguish the sea from the flood waters."
President Benigno Aquino called for the public's co-operation, warning that the government did not have "infinite capabilities" to deal with the natural disaster.
People are said to be stranded in homes all over the city.
Soldiers, police and volunteers are trying to reach them by boat, says the BBC's Kate McGeown in Manila. But some people are refusing to leave, scared their possessions will be taken by looters.
"The flooding has impacted everyone here. Even if your house did not flood - and ours didn't - the streets flooded badly and so much of Manila has been impassable and people have been stranded," Julie Green, an Australian currently living in Manila, told the BBC.
"All businesses have been closed except for 7-11s and some sari-sari [convenience] stores. Everyone's stocks are getting quite low now so you have to wake up early and battle the rains to get some food.
"It rained hard again all last night, but it seems now that the rains might have abated. People are starting to come out again."
Officials have warned that more rain is expected, however, and are urging people to consider their safety first.
Manila and the northern Philippines have been hit by severe weather since Typhoon Saola struck just over a week ago, killing more than 50 people.
The government is better prepared this time than when typhoons hit the country previously - tens of thousands of people have been evacuated, says our correspondent.
Typhoon Ketsana hit the Philippines in September 2009, causing flooding that killed more than 400 people and Typhoon Nestat and Nalgae struck two years later, leaving more than 100 dead.
The current rain and floods are said to be the worst to hit the country since 2009. However, the state weather bureau has said that weather conditions may get better later this week.