MANILA, DEC. 9 (WEDNESDAY) -- A powerful car bomb exploded last night outside Manila's bustling international airport, injuring at least four persons and shattering several glass panels in a wing of the terminal building.

Less than an hour later, a second bomb exploded next to a public utility pole in the financial district of Makati, shattering glass in a shopping center but causing no injuries.

This morning, a third bomb was found and defused outside the Asian Institute of Management, also in Makati. Police officials said the bomb consisted of 20 sticks of dynamite, a detonator and a timing device inside a shoe box wrapped with Christmas paper and left in a shopping bag.

The bombings occurred one week before five Southeast Asian leaders are due to arrive here for a summit meeting. The airport blast marked the first such attack there since President Corazon Aquino came to power in February 1986.

Airport manager Aurelio German, when asked if the bombing was aimed at disrupting the summit, replied, "Definitely."

Yesterday morning, military officials had thrown an extraordinary security cordon around the summit site, the Philippine International Convention Center, and the Plaza Hotel where the leaders of Singapore, Malaysia, Brunei, Thailand and Indonesia will be staying.

No one claimed responsibility for last night's bombings. Most speculation centered on several extremist groups, including renegade right-wing military officers who staged a bloody coup attempt in August, communist insurgents who have launched a new campaign of urban terrorism and members of the Japanese Red Army.

The Red Army is a radical group that government officials suspect may have infiltrated the country because Japanese Prime Minster Noboru Takeshita will be visiting Manila during the summit meeting.

The Philippines is determined to host the summit -- only the third such meeting of the Association of Southeast Asian Nations, or ASEAN -- partly to demonstrate that Aquino is in control and on top of the security situation. But the other ASEAN leaders have demanded extraordinary security measures.

Indonesian officials insisted that President Suharto be allowed to bring his own bullet-proof limousine and requested permission for Indonesian Navy frigates to dock in Manila Bay near the summit site in case of an emergency evacuation.

Because of security concerns, the meeting has been shortened from three days to one and a half. Airport welcoming ceremonies have been scrapped and the leaders will be whisked to the Plaza Hotel to be met by Aquino. All official functions at Malacanang palace have been canceled because the leaders were afraid of making the trip between the summit site and the palace several miles away.

Last night's blast at the airport, shortly after 7 p.m., apparently came from a bomb planted in a car parked on a concrete ramp leading to the main departure area, which is on the third floor of the terminal building. Only the car's two front wheels, steering wheel and dashboard were left intact.

The front of the departure area consists of a series of plate-glass windows, and those in the vicinity of the car bomb were shattered, with glass and debris spread up and down the ramp for several yards. News agencies quoted several witnesses as saying the blast shook the terminal building and could be heard in residential neighborhoods surrounding the airport. The blast reportedly sent shocked passengers scrambling for cover.

The wounded persons, all Filipinos, reportedly were sitting in an airport restaurant whose windows face the ramp where the car was parked. German said only one of the four was seriously injured, with facial burns.