The city logged its highest daily rainfall in August since records began in 1884, at just over 355 millimetres recorded at the Hong Kong weather observatory's headquarters at 2:00 pm (0600 GMT).

The financial hub has suspended school classes and opened temporary shelters, and some hospital services such as outpatient clinics have also been affected.

The observatory said the "black" rainstorm warning -- meaning downpours exceeding 70 millimetres in an hour -- would remain in force at least until 5:00 pm. It was hoisted late on Monday and again in the pre-dawn hours.

More than 9,600 cloud-to-ground lightning strikes were recorded between 5:00 am and noon, according to the observatory.

The weather service warned on Tuesday afternoon that "persistent rainstorm will cause serious road flooding and traffic congestion".

An AFP reporter saw some small boats had capsized off the coast of the eastern Tseung Kwan O district.

Cars were seen nearly submerged at an outdoor parking lot nearby.

The city's number two official Eric Chan urged the public to stay alert and said employers should consider flexible working arrangements that prioritise employee safety.

Emergency room service at the Queen Mary Hospital was affected for around two hours due to "severe flooding" on some roads, health authorities said.

Public transport slowed to a crawl in some districts and a handful of subway station exits were closed.

Hong Kong's airport saw delays to around 100 flights as of Tuesday morning, with one departure and one arrival flight cancelled, according to the airport authority.

The Drainage Services Department said it identified 25 cases of flooding as of Tuesday afternoon.

Neighbouring Chinese tech hub Shenzhen issued a "red" rainstorm warning earlier in the day, the first time since 2018, according to media reports.

Tuesday's "black" rainstorm warning in Hong Kong was the fourth in the span of just over a week, beating the previous record of three such warnings in a year.

Scientists warn the intensity and frequency of global extreme weather events will increase as the planet continues to heat up because of fossil fuel emissions.

China is the world's biggest emitter, though it is also a global renewable energy powerhouse aiming to become carbon-neutral by 2060.