Twenty-four people were injured Tuesday when a crowded escalator in a metro station in southern China's economic hub Shenzhen suddenly started moving in reverse.

The accident happened at 8:49 a.m. on a moving staircase at Guomao Station, said Chen Qi, deputy general manager of the Shenzhen Metro Operating Company.

The injured - including a pregnant woman - were taken to hospital where they are in stable conditions.

The metro company and the escalator manufacturer are investigating the cause of the malfunction.

One of the injured, a man surnamed Huang, told Xinhua the escalator started running in reverse at a high speed as more than 70 passengers were standing on it.

"I heard a crack and then the escalator that was moving up suddenly began going down. We were all unprepared," Huang said.

He said only a few people managed to escape by running upwards and that most were thrown backwards.

Use of the escalator was suspended after the accident. Normal operations at the metro station resumed at 10 a.m.