GENERAL SANTOS CITY, Philippines -- Four people were killed and 30 injured Wednesday in a bomb blast at the gate of a tuna canning factory in the southern Philippine city of General Santos, police and a media said.

City police chief Roberto Po said the explosive was a home-made bomb and went off at past 6 p.m. outside the main gates of Philbest Canning in the village of Bawing.

"The injured have been rushed to hospitals in the area," he said.

DZMM Teleradyo reported that four were killed while 30 others were injured in the blast, which occurred as the firm’s workers were going home.

No one claimed responsibility for the blast.

"We are still investigating whether this is an extortion attempt or labor-related," Po said. He said he was also not ruling out an attack by Muslim radicals.

Police said they had not established a motive for the bombing.

Local media described scenes of panic after the blast, with the dead and injured littering the street.

Philbest cannery was not commenting on the bombing.

Known as the tuna capital of the Philippines, General Santos, on the southern island of Mindanao, is home to some of the biggest tuna canning factories in Southeast Asia. Six of the country's eight tuna canneries are located in the city.

Established in 2001, Philbest employs 1,500 workers and is one of the biggest employers in the southern port city.

Police Wednesday night said they were not ruling out possible business rivalry as a motive.

The last major bombing in General Santos was in January 2007 when six people were killed when a home-made bomb exploded near a lottery vendors stall.

Over the years the city has been rocked by a series of bombings some of which have been blamed on the Al-Qaeda-linked Abu Sayyaf.

Though largely Catholic, General Santos is bordered by Muslim-dominated areas. Several Islamic groups are fighting in the region for independence, but the largest rebel group, the Moro Islamic Liberation Front (MILF), is in peace talks with the government. Aquiles Zonio, Inquirer Mindanao with Reuters and Agence France-Presse