The leader of the Philippine political opposition said today he had been forcibly prevented last Sunday from meeting the airplane that brought Benigno Aquino Jr. to Manila, where he was assassinated shortly after landing.

Salvador Laurel, chairman of the 12-member opposition coalition known as the United Nationalist Democratic Organization, said he had been given permission by airport authorities to greet Aquino at the plane door, but instead was forced to remain in a lounge area out of sight of the scene where Aquino was shot to death.

If he had been allowed to greet Aquino as planned, he would have been able to observe what happened, Laurel said in an interview.

Aquino, the strongest political challenger to President Ferdinand Marcos, had been accompanied by several reporters on the plane, but they and other passengers were not allowed to follow him when he was taken away by three security officers who met him at the plane.

Laurel's account explains, in part, why few eyewitnesses have surfaced to provide an independent description of how Aquino was killed. The government has said he was shot from behind by an unidentified gunman masquerading as an airline maintenance crewman.

Thousands of people shouting and waving banners lined the 65-mile route from Manila to Concepion early Saturday as Aquino's body was returned to his hometown, The Associated Press reported.

Some estimates placed the throngs that gathered beside the road in towns and cities in the hundreds of thousands. A caravan of about 100 cars led by a black hearse and a three-car police escort that had left Manila with the body of the assassinated ex-senator was joined by hundreds of other vehicles as it passed through the countryside.

"I have never seen anything like this," said Angeles City Mayor Fernando Nepomoceno as he was swept along with the crowd outside the gates of the U.S. Clark Air Base.

The Marcos government has denied involvement in the shooting.

Meanwhile, there were differing government reports today about how Aquino was to have been treated upon his arrival after three years of self-imposed exile in the United States.

Laurel said Defense Minister Juan Ponce Enrile had assured him that Aquino would only be placed under house arrest and said that Enrile hinted that he would be permitted to move around the country. Enrile had given the same assurances four days before the shooting to U.S. Ambassador Michael Armacost and Rep. Stephen Solarz (D-N.Y.), when they called on him, reliable sources said.

However, a police official today said the instructions on Sunday were to arrest Aquino and put him back on the plane he arrived in if he lacked proper travel documents.

If he had had proper documents, said Manila Police Chief Gen. Prospero Olivas, orders called for him to be taken to Fort Bonifacio prison until further instructions.

In his account of the arrival, Laurel said he had been assured by the airport manager that he and the greeting party would be allowed to meet Aquino at planeside.

But before the plane arrived, the airport manager called him and said Brig. Gen. Luther Custodio, in charge of airport security, had taken control of the airport and could not assure Laurel the promised access, Laurel said.

Laurel and others went to a lounge area to await the arrival of the China Airlines plane bearing Aquino. Then they tried to leave but a guard told him he could not exit, he said.

A few moments later, Aquino was arrested and escorted out the front exit by the guards into the elongated tube that connects a landed plane with arrival lounges.

Instead of being led through the tube, Aquino was led down steps just inside the tube's connecting door.

Seconds later, sounds of gunfire were heard inside the plane.

The police chief, Olivas, said he did not know why Laurel was denied access but said he would look into the matter.

"I know of no such deal, but I will inquire," he said.

Olivas also offered an explanation for why the three uniformed guards had appeared on the China Airlines flight to arrest Aquino when no one in the government was supposed to know which plane he was arriving on.

Olivas said the three men did not know which plane he would be on and so had routinely entered each of several planes arriving that time of day.

Asked how the gunman could have known what plane to watch for, and where to ambush Aquino, Olivas said he thought the man probably had followed the three uniformed men on each of their trips to several planes and waited beside each one until Aquino was finally brought down.

Olivas said his investigation still has not shown how an armed man could penetrate stiff airport security and get to the bottom of the stairway Aquino was required to descend.

Thousands of students rallied at universities in Manila to protest Aquino's assassination, The Associated Press reported. About 3,500 students, joined by Aquino's 23-year-old son, Benigno III, marched about the campus of the University of the Philippines in suburban Manila. Two thousand massed at Far Eastern University downtown, where student leaders said 15 persons were detained briefly by university authorities.