A power cut has hit more than half the population of the Philippines, leaving thousands of passengers stranded on the light railway system in Manila and halting trading on the Philippine Stock Exchange.

We have notified our forces to be vigilant against groups who might take advantage of the power outage

Military spokesman Colonel Jose Mabanta

More than 40 million people were left without electricity for several hours in the capital, Manila, and the rest of the main island, Luzon.

A spokesman for the Philippine National Power Corporation said a line fault had apparently caused the brownout, as electricity cuts are popularly referred to in the Philippines.

Power had been restored to half the affected areas by mid afternoon, but large areas of the capital Manila were still without electricity.

The military denied rumours that the outage was caused by sabotage. President Gloria Macapagal Arroyo is on a state visit to Japan, and rumours of coup plots have been frequent during her 16-month tenure.

"We would like to assure the people that this blackout is not in any way connected or instigated by any threat group," said military spokesman Colonel Jose Mabanta.

Vigilant

"We have notified our forces to be vigilant against groups who might take advantage of the power outage as a precautionary measure."

Energy Secretary Vincent Perez told a radio station that the power cut was caused by the tripping of transmission lines between Santa Rosa and Binan towns in Laguna province, just south of Manila.

"It's like a stone thrown into a river, creating a big wave, which caused instability in the whole grid," he explained.

State-owned National Power Corporation (Napocor) has already started restoring power in some areas and hopes to bring back power to the whole of Luzon by 1600 local time (0800 GMT).

The Philippines last suffered a major blackout in January, which was also caused by a power trip. Napocor president Ronald Quilala said the authorities are investigating the cause of the transmission line failures.