A bomb explosion outside a department store in the southern Philippines has killed 13 people and injured dozens.

A series of explosions ripped through the southern Philippine city of General Santos, with the most deadly in the mid-afternoon outside a busy department store crowded with shoppers.

Philippine police say the first explosion was followed by an anonymous call claiming 18 bombs had been placed in the city. Two other bombs went off soon after near the Radio Mindanao Network office and another at a bus terminal. But there were no injuries reported from these explosions.

General Santos, about 1,000 km. south of the Philippine capital of Manila, is a largely Christian city among the predominantly Muslim southern provinces of Mindanao.

Police reported the first blast in front of the store left shattered glass, and blood spattered around the parking area. They said most of the casualties appeared to be pedicab taxi drivers, shoppers and bystanders.

The police chief for the central Mindanao region, Senior Superintendent Bartolome Baluyot, said two other bombs were discovered under a parked truck in front of the store and were being detonated by the police bomb squad.

In similar blasts in the past, police have suspected the Muslim rebel Abu Sayyaf group, as well as the fundamentalist Moro Islamic Liberation Front.

General Santos is 220 kilometers from Basilan Island, where the Philippine Army, with the help of U.S. special forces, is carrying out operations aimed at halting Abu Sayyaf activities.

The group, known for its hostage taking and killings, is holding an American couple and a Filipina nurse in the jungles on Basilan Island.