President Ferdinand Marcos said tonight that the assassin of his longtime rival Benigno Aquino Jr. was a "professional killer" who may have had accomplices, but he said the man has not yet been identified.

Marcos, at a hastily called news conference on Manila television, said one of the "theories" being investigated is that the slaying was plotted by Philippine communists who wanted to embarrass his government.

The president also asserted that there is no truth to rumors that government agents could have prevented the killing. He insisted that they had tried to protect Aquino with their own bodies.

Aquino, the main opposition leader, was shot to death yesterday when he arrived at the Manila airport, ending a three-year, self-imposed exile. He had been expected to help lead the opposition campaign for the National Assembly elections next year.

Marcos' sudden appearance on television was in part an effort to douse some of the many rumors swirling around this jittery city and in part a counterthrust against opposition leaders who today called on him to provide answers to questions surrounding the slaying.

One opposition leader, Salvador Laurel, said earlier today that Marcos must supply answers or "it could be the end of his regime."

Laurel, an assemblyman who heads the 12-member United Nationalist Democratic Organization, in particular singled out the fact that the killer was able to get into a restricted airport zone patrolled by government police. "We question the mysterious circumstances in which the assassin was able to enter the restricted area," he said.

In statements at Aquino's suburban home, Laurel said Aquino's death would unite peaceful opponents of the government, Reuter reported.

He said, however, that "it remains to be seen" whether the peaceful opponents would unite with the violent dissidents. It is possible, he said, that some will "go to the hills," as have the Communist guerrillas in rural areas.

Asked what his organization would do, Laurel said, "We are resolved we are going to the people and tell them what's going on. We're not just going to talk. We're going to move. There should be less talk. We will have to move quietly with less rhetoric."

Former senator Eva Estrada-Kalaw, vice president of the group, said that with Aquino's death, "things will escalate."

Manila was tense but quiet today as hundreds of mourners came to Aquino's home in suburban Quezon City to file past his open coffin. It was a city rife with rumor, questions and vague accusations, but there were no indications that any of the several opposition groups would attempt to mount demonstrations against the government.

However, Malacanang Palace, the presidential residence, suggested a state of siege as a number of troops armed with automatic weapons and at least one armored personnel carrier took up positions on the grounds. There were rumors that martial law would be proclaimed, but Marcos tonight called that "ridiculous."

Adding to the eerie atmosphere was a power failure that shut off electricity to most of metropolitan Manila and left downtown areas almost totally dark. A transmission line failure was blamed.

There also were reports that one of the more powerful members of Marcos' government, Defense Minister Juan Ponce Enrile, was under arrest. To dispel that rumor, Enrile appeared smiling on television with Marcos tonight.

Marcos said such rumors were the work of "vultures" in the opposition ranks who wanted to use Aquino's death to stir up antigovernment hatred. "I appeal to the opposition to help us maintain calm, peace and order," he said. "It will serve no one any good to exacerbate feelings."

The government's failure so far to identify the killer has added greatly to the uneasiness here. Security officials yesterday killed a man at the airport who they said had shot Aquino as the opposition leader was being escorted by officials into custody. Police said that tests showed the alleged assailant had fired a gun.

Marcos said the killer was so professional that he was able to move close enough to Aquino to put the muzzle of his .357 Magnum only 18 inches from the victim's head.

In a statement distributed by the Philippine News Agency, Gen. Prosper Olivas, the chief of the Manila police who is conducting the government investigation of Aquino's death, said that the gunman was between 30 and 35 years old and died of eight gunshot wounds in several parts of the body. Tests showed no trace of alcohol or drugs in his system, he said.

Olivas said authorities did not know how the assailant got into the airport area. He said 14 security guards from the airport who were assigned to provide security for Aquino are being held pending the investigation. They have been subjected to paraffin tests to see whether they fired their weapons, and their firearms have been confiscated, he said.

In his television remarks, Marcos said that the reports in early August of a planned assassination attempt on Aquino included hints of a broad conspiracy. "This is the reason we begged him not to come back home now," he added.

At one point, Marcos suggested the conspiracy might have been plotted by the communist guerrilla movement in the Philippines, the New People's Army. At another point, however, he said some reports hint that it may have been planned by persons who were friends of men who had been mysteriously "liquidated" after testifying against Aquino in a trial a decade ago. Then Marcos added, "Of course, these are all just theories."

Marcos accused "high-ranking" officials of an unidentified foreign government of helping Aquino return to the Philippines using fake documents, The Associated Press reported. Marcos' government had refused to issue Aquino a passport to replace his expired travel papers, claiming that certain groups were out to kill him upon his return. Aquino returned using a passport with an assumed name.

Aquino had told an AP reporter that he had acquired a valid passport from the Philippine Embassy in Washington with the help of an unidentified embassy officer. Family members said he boarded the plane under the name Bonifacio, after a Philippine national hero who was killed by his political rival during the struggle for independence from Spain.

Aquino told United Press International Sunday that he had the help of high-ranking officials in Taiwan and several other Southeast Asian countries in his bid to come home.

His family scheduled the funeral for Sunday, UPI reported.

Marcos had been in self-imposed seclusion, ostensibly writing a book, for several days before the assassination. This had fanned rumors that he is seriously ill.

Tonight, the president scoffed at those rumors and said jokingly, "I am here. I am ready to wrestle with anybody."

He called these stories "part of the hate campaign" being waged against his government.

"I appeal to the people to maintain calm," he added.

The rumors that martial law would be imposed led to panic buying of food in some sections of Manila this afternoon.

Imelda Marcos, the president's wife and herself a powerful government official, appeared with him on television tonight to assure the public that ample supplies of food are available.