Hong Kong was flooded by the heaviest rainfall in nearly 140 years on Friday, leaving the city's streets and some subway stations under water and forcing its schools to close.

Just across the border, authorities in China's tech hub Shenzhen recorded the heaviest rains since records began in 1952.

Climate change has increased the intensity of tropical storms, experts say, with more rain and stronger gusts leading to flash floods and coastal damage.

The heavy rains in Hong Kong started on Thursday and in the hour leading up to midnight, the city's weather observatory recorded hourly rainfall of 158.1 millimeters at its headquarters, the highest since records began in 1884.

On Friday afternoon, the Hospital Authority said at least 110 people were hospitalized due to injuries, with four in serious condition.

A man was found unconscious off the shore of western Hong Kong Island and declared dead at the hospital, though authorities were still investigating if the death was flood-related.

The city's highest rainstorm warning level, "black", was hoisted for a record-breaking 16 hours before being lowered at 3:40 pm Friday, with rainfall mostly easing by late afternoon.

"It's absolutely shocking," said Jacky, 52, who lives in the Wong Tai Sin district with his elderly parents. "I don't remember floods ever being this bad in our district."

"The bottom floor of the mall is completely flooded, the water level is higher than the storefronts… it's turned our day into chaos," he added.

Authorities issued flash flood warnings, with emergency services conducting rescue operations in parts of the territory.

"Residents living in close proximity to rivers should stay alert to weather conditions and should consider evacuation" if their homes are flooded, the observatory said.

It also warned of potential landslips, telling motorists to "keep away from steep slopes or retaining walls".

Hong Kong's stock exchange cancelled all trading sessions on Friday.

'Once in a century'

Hong Kong Chief Secretary Eric Chan described the deluge as "a once-in-a-century heavy rainstorm", adding that extreme conditions would continue until midnight local time (1600 GMT).

"It's like putting four bathtubs of water into one bathtub… it will spill," Chan said at a press conference, when asked if the government had done enough to prevent flooding.

The Hong Kong Observatory said it recorded more than 600 millimeters of rainfall at its headquarters over 24 hours — roughly a quarter of the city's annual average.

Earlier in the day, taxis struggled through flooded roads as commuters attempted to make their way to work, with some cars stranded in the deluge.

"It felt like the whole neighborhood was isolated by the floodwater. One of the underground car parks is totally under water," Olivia Lam, who lives on the eastern side of Hong Kong Island, told AFP.

"The water was almost waist-deep outside my building, and that's not the worst (case) in the neighborhood."

An AFP reporter saw boulders and mud from a landslide block off a two-lane road in the Shau Kei Wan district, with mud also spilling over into a nearby basketball court.

Residents of a public housing block just 30 meters from the landslide lined up with buckets to collect fresh water after the building's supply was disrupted.

Roads were also flooded on the island of Lantau, where rivers swelled over their banks.

Southern China was hit the previous weekend by two typhoons in quick succession — Saola and Haikui — though Hong Kong avoided a feared direct hit.

Tens of millions of people in the densely populated coastal areas of southern China had sheltered indoors ahead of those storms.

Hong Kong's weather observatory said the latest torrential rain was brought by the "trough of low pressure associated with (the) remnant of Haikui".

Authorities suspended schools and cargo clearance services on the city's border with Shenzhen were paused.

The border disruption came hours after Hong Kong authorities announced that Shenzhen was preparing to discharge water from its reservoir, which they said could lead to flooding in northern parts of the city.

Hong Kong's subway operator said there was a service disruption on one of its lines after a station in the Wong Tai Sin district was flooded.

A handful of other stations were also affected by the rain.

Footage posted on social media showed a subway train not stopping at Wong Tai Sin station, which had floodwater on its platform.

The flooding could cost Hong Kong at least $100 million, according to a Bloomberg Intelligence estimate, compared to $470 million in damage when the city was hit by typhoon Mangkhut in 2018.