An aerial photo shows rescuers work at the site of a collapsed road section of the Meizhou-Dabu Expressway in Meizhou on Wednesday. AP

Eighteen cars fell down a slope after a 17.9-metre long section of the highway collapsed, according to a statement from authorities in Meizhou city in Guangdong province. The incident occurred around 2am.

The death toll had risen to 24 by Wednesday afternoon, according to China's official Xinhua News agency.

Parts of Guangdong province has seen record rains and flooding in the past two weeks, as well as hail. Some villages in Meizhou had flooded in early April, and the city had seen heavy rains in recent days.

Witnesses told local media they heard a loud noise and saw a hole open up several meters wide behind them after driving past the section of the road just before it collapsed.

Rescue workers have taken 30 people to the hospital, state broadcaster CCTV reported.

An aerial photograph published by state broadcaster CCTV showed wrecked vehicles lying in a deep muddy pit where the highway once ran.

Dozens of emergency vehicles and cranes congregated along the intact section of the road, which bisects a steep, wooded slope.

Videos circulating on social media — apparently filmed before dawn — showed flames and smoke emanating from the pit.

"You can't go any further," a man is heard saying in one video, adding that parts of the road had given way. AFP was not immediately able to verify the videos.

CCTV said the collapse was a "natural geological disaster... (that occurred) under the impact of persistent heavy rain."

Authorities have dispatched around 500 people to the site to help with the rescue operation, CCTV reported.

Agencies