Cotabato City, Philippines  - Four people were injured when a homemade bomb exploded at a passenger bus terminal in the southern Philippines, police said Tuesday.

Chief Superintendent Felizario Serapio, a regional police chief, said the bomb exploded Monday evening at one of the gates of the Weena Bus Co terminal in Cotabato City, 930 kilometres south of Manila.

"We believe the motive in the attack has something to do with extortion," he said.

Serapio said that immediately after the bombing, a man called up the bus owner and demanded 1 million pesos (23,529 dollars) as protection money.

The Weena bus company has been a target of bomb attacks blamed on criminal syndicates demanding protection money in recent years.

On April 29, a homemade bomb exploded at its terminal in Midsayap town in nearby North Cotabato province. No one was injured in the explosion, but three buses were damaged. (dpa)