SINGAPORE (Reuters) - A leak of refrigerant gas from a train's air-conditioning system caused what looked like smoke to billow from an underground station in central Singapore on Monday, train operator SMRT said.

There were no injuries, police said.

Videos on social media showed commuters rushing out of a train at the Tanjong Pagar station, covering their noses to protect themselves from grey gas. Some of them were seen taking pictures on their phones.

The gas triggered alarm after Indonesian police arrested six suspected Islamist militants on Aug. 5 on suspicion of plotting an attack on Singapore.

Margaret Teo, head of corporate marketing and communications for train operator SMRT Corporation Ltd, said there had been a leak of Freon gas.

"The discharge of Freon into the tunnel reduced visibility at parts of the station when the platform screen doors were opened for passengers to exit the train," Teo said in an e-mailed statement.

"SMRT staff switched on the tunnel ventilation system immediately to disperse the odourless gas," she said, adding that train services were not affected.

(Reporting by Masayuki Kitano and Marius Zaharia; Additional reporting by Aradhana Aravindan; Editing by Nick Macfie)