A bomb ripped through a bus in suburban Manila late Friday, killing at least three people and injuring 23 others, hours after a grenade blast in the capital's financial district and a day after two deadly bombings in the southern Philippines.

There was no immediate claim of responsibility for the bus blast, but officials have said the Al Qaeda-linked Abu Sayyaf group was the most likely suspect for Thursday's noontime bombings in downtown Zamboanga city that killed seven people and injured more than 150.

The bus explosion took place at 10 p.m. on the EDSA highway, one of the capital's main thoroughfares, in Quezon City, despite tightened security following the earlier attacks.

"I was sleeping, then there was a very loud explosion," teenage student Merlyn Villareal, who was aboard the bus but was not injured, told GMA7 television as she fought back tears. "There was chaos, and I was pinned down. I was kicked around and found myself outside the bus."

The explosion in the back of the blue Golden Highway company bus ripped off its roof and part of its sides and sent debris flying 20-30 yards away. Two hours later, workers still had not managed to retrieve one badly mangled body from the vehicle, which had roughly 50 to 60 seats.

"This is the handiwork of people with evil minds," national police operations chief Vidal Querol said.

Napoleon Castro, a Quezon City police official, said officials believed the person who brought the bomb aboard the bus was among the dead.

"It was a very powerful bomb for this bus to be wrecked like this," Castro said.

Traffic was backed up more than a mile on the highway, which is several lanes across where the explosion took place.

Querol said there were no immediate suspects and that investigators were gathering fragments from the bomb to analyze.

National Security Adviser Roilo Golez said the way bombing was carried out "is very similar" to the Dec. 30, 2000, simultaneous bomb attacks in Manila that killed 22 people and were linked to Jemaah Islamiyah, a shadowy, Southeast Asian-based Muslim group suspected of links to Al Qaeda.

"The only conclusion we can make is that we should be on the alert and that the public should help," Golez said.

He said top government officials will meet Saturday to assess the situation.

No one has been charged for the 2000 bombings that hit several public facilities, including a train station. But Fathur Rohman Al-Ghozi, an Indonesian, told police he helped plan them. He pleaded guilty to explosives possession charges in April and was sentenced to 10-12 years in prison.

He was arrested in January and led police to a buried stash of 1.2 tons of TNT that allegedly were to be used for attacks on Western targets in Singapore. Philippine prosecutors said Monday they have strong evidence to charge two foreigners and two Filipinos who allegedly provided money to buy the explosives.

Like Al-Ghozi, the four are believed to be members of Jemaah Islamiyah.

The country, already jittery, was put further on edge when a grenade exploded early Friday in Manila's financial district. The grenade caused no injuries and slight damage to one vehicle. A second unexploded grenade was found nearby.

While officials said they believed that explosion was unrelated to terrorism, Manila Mayor Lito Atienza imposed a night curfew in the capital for everyone under age 18.

Golez said top government officials will meet Saturday to assess the situation following the bus blast.

Earlier Friday, President Gloria Macapagal Arroyo visited the Zamboanga bombing sites, saying the country's bomb attacks have gone from "bad to worst" and urged Filipinos to help fight terrorists.

"Terrorism cannot survive for long in an unfriendly environment," Arroyo told reporters. "Let us give terrorism the unfriendly environment."

Defense Secretary Angelo Reyes speculated that Zamboanga bombings may have been staged by the Abu Sayyaf or the Muslim separatist Moro Islamic Liberation Front to either retaliate for or divert attention from simultaneous military offensives against the guerrillas on two fronts in the south.

The Abu Sayyaf recently threatened attacks in retaliation for an ongoing military offensive against it, and has been blamed for an Oct. 2 bombing in Zamboanga that killed four people, including an American Green Beret.

Some 260 American troops remain in Zamboanga in the violence-wracked southern Philippines following a six-month U.S. counterterrorism training exercise aimed at helping Filipino troops fight the Abu Sayyaf. No foreigners were believed to have been injured in Thursday's bombings.