At least 24 people were killed when a section of a highway collapsed early Wednesday, the start of a five-day long Labour Day holiday, in China's rain hit Guangdong province.

The cave-in occurred at around 2am in Dabu County of Guangdong’s northern Meizhou City. Nearly 18 metres of the mountainous highway crumbled into the forested slope below, trapping 20 vehicles and 54 passengers in all, the Hong Kong-based South China Morning Post reported, quoting a local government statement.

The official statement said 30 victims had been sent to hospital, while around 500 public security, emergency response, firefighting, health and sanitation and other workers were taking part in the rescue work.

The condition of the hospitalised patients was “not life-threatening at the moment”, the statement said, but did not specify the level of their injuries.

The reason for the collapse has yet to be declared and the matter is under investigation, the Post reported.

The accident occurred as China began a five-day break for Labour Day, one of the four major holiday and travel periods for the country when highways are toll-free and see heavy traffic.

Footage and pictures shared by local news outlets showed flames and smoke rising from the collapsed section, while charred cars were visible on the gouged-out slope. The stricken highway links Guangdong to southeastern Fujian province.

State-run Xinhua news agency reported that 31 people have been sent to hospital for treatment after a road cave-in on an expressway in south China's Guangdong Province in which 18 vehicles were trapped.

The cave-in happened around 2:10 a.m. on a section of the Meizhou-Dabu Expressway in the city of Meizhou, according to the government of Meizhou.

The expressway has been closed as the rescue is still ongoing. The cause of the accident is being probed, the report said.

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