A bomb on a packed bus in the Philippines' financial hub killed four people on Tuesday, sparking a warning from President Benigno Aquino that the country faced a raised terror threat.
Authorities said it was too early to say who was behind the explosion as the bus travelled along one of Manila's busiest roads, but Aquino ordered security forces on alert.
"We have asked the police and military and the intelligence services to review their threat assessment, with the end view of hardening all areas considered under threat," Aquino said in a nationally televised address.
"So far the investigation is ongoing, but the mechanical and electrical causes are being ruled out for this explosion. There appears to be an explosive device... we are looking at the terrorist aspect of it."
Metropolitan Manila police chief Director Nicanor Bartolome said the bomb appeared to have been placed under one of the passenger seats.
Police initially placed the death toll at two, but later revised it to four after two of the 15 injured people taken to hospital died from their injuries.
Vice President Jejomar Binay, a former mayor of the Makati financial district, arrived quickly at the scene and said it appeared to be a powerful blast.
"There's a huge hole and the debris indicates it is a bomb," Binay said, adding the legs of one injured woman had been blown off.
Police investigators were seen scouring the scene for evidence, with blood drying on the pavement.
The driver, who survived the attack unscathed, said at least two men who acted strangely quickly got off the bus just moments before the explosion.
"They were moving from one seat to another although we had no idea there was an explosive aboard," the driver, Maximo Peligro, told reporters.
In November, the US, Australian, British, Canadian, French and New Zealand governments updated their travel advisories to the Philippines to warn that terrorist attacks were likely.
Some of the warnings specifically mentioned that an attack would take place in Manila.
Aquino strongly criticised the advisories then, saying they were not based on firm intelligence, prompting some of the governments to downgrade the threat assessment.
However, Aquino admitted Tuesday that his government had received a report last year that a "Muslim secessionist group" he did not identify was planning to carry out attacks in Manila.
"We have had a report that there were terrorist groups planning the same. But the assessment at the time was that there was a lack of resources to be able to carry it out and a lack of support base," he said.
Aquino said the report mentioned plans for a bomb attack, but he did not elaborate.
The Philippines faces a range of security threats, most prominently the Abu Sayyaf Muslim militant group that operates mainly in the south of the country, and communist rebels who have been waging a rebellion for 42 years.
The Abu Sayyaf bombed a passenger ferry on Manila Bay in 2004, killing more than 100 people in the country's worst terrorist attack, according to authorities.
The Abu Sayyaf was also blamed for a bomb attack on a bus near Makati in 2005 that left three people dead.
In an incident that was not terrorist-related but nevertheless raised international concerns about security in Manila, a disgruntled ex-policeman hijacked a tourist bus in August last year.
After a day-long stand-off, police stormed the bus and killed the hijacker, but eight Hong Kong tourists also died.