Manila — The assassination of the Philippines' leading political opposition figure, Benigno S. Aquino, Jr., marks a historic turning point for the virtual one-man rule of Ferdinand Marcos.
Mr. Aquino, gunned down at the Manila airport Sunday as he returned from three years of self-exile in the United States, had planned to pull together the remaining elements of nonviolent opposition against the US-backed Marcos regime in the Philippines.
Feared by both the President and First Lady Imelda Marcos for his popularity among Filipinos, Aquino had appeared to be the only opposition figure with sufficient potential to contest the Marcos regime. Well aware of the risk he was taking in returning, Aquino's assassination now leaves a vacuum in the Philippine political opposition and possibly a strain in US-Philippine relations.
Aquino, who believed his health left him only a few years to live, had hoped to persuade Marcos to restore full democracy before the president left the scene , so as to prevent various factions - including the military and Mrs. Marcos - from further weakening this militarily strategic American ally in a possible fight for power.
If it is discovered that Marcos had a hand in the killing, this could make Aquino a rallying point as a political martyr, jeopardizing the 17-year-old Marcos government and casting a shadow over President Reagan's scheduled visit to Manila late this year.
So far, the killing is a mystery. The assassin's identity remains unknown, but as of this writing the Marcos regime is under strong suspicion by many eyewitnesses and critics.
When Aquino's aircraft, a China Airways flight from Taipei, stopped at the arrival gate in Manila early Sunday afternoon, three military officers boarded the plane and escorted Aquino off. One American eyewitness says Aquino's did not resist the officers.
A minute later, the witness continued, a flurry of shots was heard, and Aquino was put in a van by the military. The van is thought to belong to the Air Force's Aviation Security Command.
The alleged assassin, according to the government, was also killed in the shooting. The government claimed he had disguised himself as a maintenance worker to avoid security precautions.
Strangely, however, five and a half hours later, the alleged assassin's body was still lying at the airport. And by almost all appearances, the man did not look like a typical mechanic and was dressed in a way that would have been difficult to hide a large gun holster on his belt.
A second eyewitness claims that Aquino had been shot just as he was about to enter the government van, presumably from inside the vehicle. And the Japanese Kyodo press agency reports that one of its journalists on the plane saw two of the three uniformed men open fire on Aquino.
Thus, the exact circumstances of Aquino's death could affect the future political opposition to Mr. Marcos.
Aquino was returning to the Philippines in the hope of resuming a political career that had been frozen for the last ten years. When martial law was declared in 1972, he was detained and imprisoned for eight years. In 1977 he was sentenced to death by a military tribunal for subversion. He denied the charge, which alleged collaboration with the communist underground. He denied being a communist and, in fact, the communist underground here is known to view him with suspicion.
Aquino left the Philippines in 1980 with presidential approval to receive heart treatment in the US. He spent the next three years at Harvard University and, later, the Massachusetts Institute of Technology.
He announced in June that be planned to return home. Shortly afterward, the Philppine government asked him not to return as ''groups are planning to assassinate you when you come to the Philippines.'' The government refused to renew his expired passport, and threatened to fine any airlines that brought him into the country. The government had, however, never disclosed the identity of those who planned to kill Aquino.
Aquino is believed to have been traveling with papers issued by US immigration authorities. The documents are thought to certify that Aquino had entered and left the US legally.
His obvious aim, had he been able to return, was to revitalize the middle ground of Filipino politics, which had been largely lifeless and devoid of ideas since the declaration of martial law. He probably agreed with the communist underground on one point: that Mr. Marcos's rule was hastening the political polarization of the country. While this phenomenon works to the underground's advantage, it clearly worried Aquino. He would probably have been active in the national assembly elections scheduled for next year.
Most observers also feel that he was planning to run in the 1987 presidential elections. President Marcos, in fact, is reliably reported to have said privately last month that his main candidate in 1987 would be Aquino.
The effects of Aquino's murder on the legal opposition is difficult to gage. On one hand, Aquino was returning to a greatly changed political scene. At least one of the men formerly close to him, Aquilino Pimentel, currently under house arrest in the southern Philippines, has started his own party. When contacted last week, he was not enthusiastic at the prospects of Aquino's return. Other opposition politicians probably felt the same.
On the other hand, Sunday's killing could lead the opposition to question their participation in thepolitical process. Opposition leader Salvador Laurel said Sunday before the assassination, ''If the government continues to rig elections, we may just dismantle the opposition and leave the field to the two violent groups - the communist New People's Army and government.'' Aquino's death may well prompt such a response.
The opposition is also likely to raise questions about President Reagan's visit scheduled here late this year. Aquino is said to have told friends that he had planned his return close to the date of the Reagan visit because ''no one would do anything to me - it would jeopardize the President's visit.''
The reception planned today at the Manila airport for Aquino did not give any indication of an immediate groundswell of support for the opposition exile. Mr. Aquino's colleagues here had predicted that 10,000 to 20,000 people would turn out. Instead only a few thousand seemed to have gathered. And these appeared detached and passive when former Senator Laurel announced that Aquino had been shot.
The government attorney in charge of investigating the incident said he expected to finish inquiries Monday.
A statement by President Marcos was read on televsion this evening. In it he said that the govenrment had ''practically begged Aquino not to return home,'' and stressed that the government had ''repeatedly and strenuously '' warned Aquino against returning for the momemnt. He also said that the normal flow of life and business would not to be disrupted by the killing. He said that he had taken measures to assure this. The President did not, however, spell out the measures.