More than a million people in and around the Philippine capital battled deadly floods Wednesday as more rain fell, with neck-deep waters trapping both slum dwellers and the wealthy elite on rooftops.

Monsoon rains that have pounded Manila for more than a week eased slightly overnight, but the government said between 60 and 80 percent of the megacity remained under water, and the bad weather was likely to persist throughout the day.

"The roads in some areas are like rivers. People have to use boats to move around. All the roads and alleys are flooded," civil defence chief Benito Ramos told AFP.

The death toll in Manila and nearby provinces rose to 20 on Wednesday, including nine members of one family who died in a landslide.

The worst hit parts of Manila were mostly the poorest districts, where millions of slum dwellers have built homes along riverbanks, the swampy surrounds of a huge lake, canals and other areas susceptible to flooding.

In Santo Domingo, a creekside shantytown, mother-of-three Anita Alterano recounted how her family escaped the floods that submerged their one-storey home by walking over the roofs of houses until they reached high ground.

"We initially just decided to climb up on the roof where we were safe but wet. We waited for rescuers but it took so long for anyone to notice us," said Alterano, 43.

"So we got a rope, I tied myself to my husband and my children, we clambered from roof-to-roof... until we reached a school. But the problem is we have no water and food."

Alterano spoke to AFP while wading through the waist-deep water trying to get back to her home to salvage some clothes and food.

Nearby, rescue workers from the local fire brigade tried to retrieve other residents still stranded on their roofs. But the fire brigade had only one, non-motorised aluminium dinghy.

Some of Manila's richest districts were also affected, including the riverside community of Provident where water had inundated the ground floors of three-storey mansions.

Inside the gated village of about 2,000 homes, rescue workers on a motorised rubber boat drove past submerged luxury cars to retrieve children and the elderly from rooftops.

Across Manila and surrounding areas, 1.23 million people were affected by the floods, forcing 850,000 of them to seek help from rescue workers, according to the National Disaster Risk Reduction and Management Council.

Nearly 250,000 of them were sheltering in schools, gymnasiums and other buildings that have been turned into evacuation centres, while others were staying with relatives and friends, the council said.

Despite the chaos which paralysed much of the city on Tuesday, the government ordered its employees and private sector workers back to their jobs, while the stock market resumed trading.

Twenty people were confirmed killed in the latest barrage of rain that began on Monday, the council said, after two other provinces reported their first flood-related deaths.

They brought the number of people killed by the monsoon rains across the Philippines since late July to 73, according to authorities.

The Philippines endures about 20 major storms or typhoons each rainy season, many of which are deadly.

But this week's floods in Manila, a sprawling city of 15 million people, were the worst in the capital since 2009, when tropical storm Ketsana killed more than 460 people.

The typhoons and storms typically start in the warm waters of the Pacific Ocean, then roar west towards the Philippines and onwards to other parts of southeast Asia, or further north to Taiwan, mainland China and Japan.

In China, authorities moved more than 1.5 million people out of the path of Typhoon Haikui before it slammed into the east coast on Wednesday morning.

China's financial centre Shanghai avoided a direct hit, but flights and some train services were suspended there and officials warned the biggest impact might be from rainfall later on Wednesday.

Haikui was the third typhoon to hit China in a week, with 23 people dying in the barrage of storms, according to Chinese state media.