The crude device attached to a parked tricycle outside the Philbest plant in General Santos City exploded late on Wednesday as about 1,500 workers were changing shifts.

One person died of injuries overnight, bringing the death toll to four, said Senior Superintendent Robert Po, the city's police chief.

Among the dead was an 11-year-old boy who was selling fried bananas outside the factory, Radio DZRH reported.

At least 27 others were hurt, most of them cut by flying glass and debris, said Dr Bing Aquino of St. Elizabeth Hospital.

Police were investigating the possibility that the blast, caused by a device made from a mortar shell, was linked to a labor dispute or an extortion attempt, Superintendent Po said.

A more powerful bomb could have caused more casualties, he added.

Al-Qaeda-linked extremists were blamed for January 2007 bombings in General Santos City and two other southern towns that killed seven and wounded 44 people.

Extortion gangs also are active in the region.

General Santos City, a bustling tuna exporting centre on the main southern island of Mindanao, is about 1,000 kilometres south of Manila. -- AP