The explosion, which sparked a massive fire, took place on an expressway in the provincial capital of Guangzhou around 5.15 am local time, a spokesman with the city's public security bureau said.
He said 20 people were killed in the explosion.
State-run Xinhua news agency reported that the 14 others who were injured in the incident have been hospitalised.
The fire caused by the blast was brought under control around 8.30 am local time, according to firefighters.