Tens of thousands of Filipinos formed a procession with the coffin of slain opposition leader Benigno Aquino Jr. here today in one of the biggest public displays of antigovernment sentiment in the history of the Philippines.

The crowd, by most estimates, was the biggest since Pope John Paul II's visit here in 1981 and surpassed the turnout for the 1957 funeral of popular former president Ramon Magsaysay.

At the head of today's procession was Aquino's younger brother, Agapito, whose prominent presence gave the first clear indication that he intends to carry on Benigno Aquino's work as leader of the opposition against President Ferdinand Marcos.

Tens of thousands of people wearing black ribbons of mourning were strung out for a mile in the slow procession that followed the hearse bearing the body of the slain politician, while out in front, making his first political bows, was Agapito Aquino.

A crowd of admirers surged around him in the middle of Quezon Avenue and the procession slowed as he shook hands and chatted. With a broad smile and a sweep of his arm that took in the large following, he exclaimed, "This shows us that we must fight for the ideals that my brother died for."

Agapito Aquino's presence at the head of the file was evidence that for the moment at least he has been thrust forward in a new and unaccustomed political role.

Several leaders in the ranks of the opposition to Marcos had urged him to step forward shortly after his elder brother was shot to death at Manila's airport Sunday. Benigno Aquino was returning from three years of self-imposed exile in the United States.

Agapito's prominence, they said, would keep the Aquino name alive and perhaps provide a rallying point for the opposition when it fields a slate of candidates for the National Assembly elections in May. Famous names mean a lot in politics here and, moreover, he has much of the ebullient charm that supporters of his brother found appealing.

Known to most Filipinos by the nickname "Butz," he has not been conspicuous in politics. He is a successful merchant of a line of plastic chairs and has achieved a modest success as an occasional movie actor. At a lunch four years ago, when his brother was languishing in a Fort Bonifacio prison cell, he seemed to guests almost disinterested in the political life.

During a curbside pause, a foreign correspondent asked him if he would step forward to succeed his slain brother.

"Since I am the next brother in line," he began modestly, "they expect me to do the same things, but I am not so sure."

Then he began gesturing and speaking for the crowd that clung around him. "It is the strength of the people that will do it," he said, "but they must begin to move. If we stand up and fight we have a chance." The crowd began cheering, and Aquino waved in its direction. "When we give the signal, they will move."

Today's procession to the neighborhood Roman Catholic church was a second stage in the memorialization of the former political opposition leader, provincial governor and senator who a decade ago, before his arrest and death sentence on charges of murder and subversion, had been considered the chief challenger to Marcos for the presidency.

The body has been on display at the Aquino residence since Sunday's shooting, and more than 50,000 mourners have passed by his coffin.

This weekend, the body will be taken on a slow pilgrimage to Tarlac, where Aquino was born, in central Luzon. After two days of public viewing there, it will be returned here for funeral services Tuesday.

The procession today, under a fierce morning sun that sent temperatures up to 97 , was a mixture of politics, religion and simple good times, a blend not uncommon in the Philippines.

White-frocked priests strode solemnly beside a youth group singing "Blowin' in the Wind," which here, as elsewhere, has overtones of social protest. From time to time the marchers sang a nationalist song about a bird wanting to be free. From a loudspeaker mounted on a pickup truck came a series of chanted "Hail Marys," followed by a popular melody sung by a pop star.

On the curb a middle-aged government employe, who declined to give his name, described the reason for the large crowd.

"We all loved Ninoy so much," he said. "He was so charismatic. No one can take his place. So now they are grooming Butz. But it will take a long time."