Manila: At least three people were killed and 20 injured when an explosion tore through a packed bus in Northern Manila late on October 18, Philippine authorities said.

The blast ripped open the rear roof of the vehicle as it traversed a major highway, they said. Police said that they had no suspects.

"We have here three dead and 20 injured," President Gloria Arroyo's National Security Adviser Roilo Golez told local radio as he inspected the wrecked vehicle.

"This is a serious incident considering what occurred before it," he added.

The blast followed bomb attacks in Zamboanga in the Southern Philippines on October 17, which left seven people dead and 162 injured. Seven unexploded bombs were later found and disarmed in the area.

"The President is very much concerned about this," Golez said, adding that he had spoken to her three times by mobile telephone.

The mayor of the district, Feliciano Belmonte, issued the same casualty toll and said ambulances were struggling to reach the area through heavy traffic.

He said that the explosion hit the Golden Highway bus around 14:45 hours (GMT).

"We don't have the identities of the dead yet," said Vidal Quirol, head of the national police directorate for operations.

"We cannot say at this time who was behind this," he added.

Johnson Eleazar, who suffered shrapnel injuries, told local television from his hospital bed that he recalled praying inside the bus shortly before the blast, mindful of the Zamboanga attacks. He did not recall any explosion.