MANILA, October 18 (IslamOnline & News Agencies) - At least three people were killed and 20 injured when an explosion tore through a packed bus in northern Manila late Friday, October 18.

The blast ripped open the rear roof of the vehicle as it traversed a major highway, Agence France-Presse (AFP) quoted authorities here are saying.

"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 carried out in Zamboanga in the southern Philippines on Thursday 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 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 the explosion hit the Golden Highway bus around 10:45 pm (1445 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."

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.

"When I opened my eyes everything was blank," he added.

ABS-CBN television said rescuers saw some of the bloodied survivors crawling on the street away from the vehicle.

Taxi drivers picked up some of the victims and took them to hospital, it added.

Police said many of the casualties suffered burn injuries.

Asked if the bus attack was terrorism, Golez said it has to be investigated first.

However, he said police have told him that explosives used in Friday's blast were exactly the same as materials used in a wave of bombings that hit Manila on December 30, 2000.

The blasts, which targeted a passenger train, killed 22 people and left more than 100 others injured.

Golez said Arroyo had just convened a crisis meeting of the cabinet when the bus attack occurred.

She had also visited the blast site in Zamboanga and condoled with the victims.

He said Arroyo called another meeting with Manila's mayors for Monday October 21 "to discuss the security in Manila."

Earlier Friday , some 300 policemen patrolled major shopping malls amid fears of attacks similar to those in Zamboanga.

Six buildings across the city of 10 million also fielded telephoned bomb threats, but no explosives were found.

The security precautions in Manila followed a grenade explosion before dawn Friday that damaged a van in the financial district of Makati, but injured no one.

Another grenade was found nearby and safely detonated.