Torrential rains have pounded southern China, flooding low-lying homes and roads, choking expressways, and prompting officials to suspend classes while the record-breaking storms lingered over the region.

By Friday morning, Shenzhen, a southern Chinese commercial city next to Hong Kong, had endured nearly eight inches of rain overnight, in the most intense downpour since records began in 1952, according to the city’s official news service. Hong Kong was hit by about six inches of rainfall in several hours. The Hong Kong Observatory headquarters recorded over six inches in just one hour, the most in that spot since its records began in 1884, according to the South China Morning Post.

In Hong Kong, at least one subway station flooded, and commuters abandoned vehicles caught on roads that had turned into raging rivers. Social media video showed commuters trapped on flooded buses with ankle-deep water sloshing between the seats. Other clips captured subway riders’ shock as they pulled into the Wong Tai Sin station to find the platform submerged.The rainstorms were caused by remnants of Typhoon Haikui, which had been grinding along the Chinese coast until striking the densely populated tip of Guangdong Province, home to much of China’s manufacturing and commerce.

On Chinese social media and news sites, video footage showed torrents of muddy water washing over cars, flooded underground parking areas, and homes that had been inundated with water.

An official at a hospital in Shenzhen put out a message saying that 20 patients in its intensive care unit had been trapped in knee-high water. When contacted, a staff member at the hospital said the problem had been quickly solved and declined to give more details or his name.

Rescue services in Shenzhen evacuated or rescued over 3,000 people from flood dangers, the city’s authorities said. Officials also announced that water would be released from a nearby dam in the early hours of Friday to relieve pressure on it, and warned residents downstream to stay away from swollen rivers. But in an apparent effort to ease the public’s concerns, the city government said the dam release would be limited.

Heavy rains also pounded the coast of Fujian Province in China’s southeast, forcing around 300,000 residents to move to safer ground, according to official estimates.

Last month, some residents of Zhuozhou, a city in northern China, protested after much of the area was flooded once officials opened flood gates and spillways in flood control zones. The action was apparently meant to prevent the waters from overwhelming bigger urban areas, including Xiong’an, a new city championed by China’s leader, Xi Jinping. On Thursday, Mr. Xi visited Shangzhi, a city in China’s northeast that was also struck by the floods last month.

Classes were suspended in Hong Kong, Shenzhen and other parts of southern China on Friday. But across the region, some people returned to work, despite transit disruptions.

David Pierson and Olivia Wang contributed reporting from Hong Kong.