The assassination yesterday of Benigno Aquino Jr. is likely to radicalize elements of the moderate Philippine opposition, strengthening the hand of leftist insurgents and possibly increasing pressure on President Ferdinand Marcos, according to exiled opposition representatives and foreign policy specialists.

"Our group's attitude toward this happening will be one of restraint of our emotions," said Renato Roxas, chairman of the U.S.-based Movement for a Free Philippines. "But it might be something that will create some bloodshed."

Most opposition leaders said they felt Marcos was responsible for the assassination, which occurred while Aquino was in the custody of government security forces at the Manila airport.

A Washington specialist, citing problems the assassination could cause for Marcos, said, however, "It's very hard to speculate on who is responsible." Among the difficulties he mentioned was the possibility of a political backlash.

Roxas' group is widely viewed as moderate--striving for democratic reform and pluralism through open elections. He said "some elements" of the movement will view Aquino's assassination as a signal that the government will not allow the moderate United Nationalist Democratic Organization--a coalition of opposition groups--to mount a fair challenge to Marcos in elections next year.

"At the moment there will be disillusionment and desperation," said Raul Manglapus, an exiled moderate opposition politician. "This could create a trend toward radicalization," the former senator added.

William Christeson, spokesman for the Friends of the Filipino People, said the assassination "signals a hard line for Marcos . . . . He is not going to tolerate opposition." Lacking an electoral avenue to institute democratic reforms, Christeson and other opposition figures say the moderates may have to turn to insurgency tactics.

"The moderates will now have to think of joining the armed struggle," said Charito Planas, who was imprisoned by Marcos under martial law and ran unsuccessfully with Aquino in 1978 elections. Planas, who left the Philippines in 1980 and refuses to label herself either a moderate or communist member of the opposition, said, "There is a realization for a need of a united front" of moderate and leftist opposition groups.

The leftist opposition, known as the National Democratic Front, consists of communist and noncommunist insurgents who have endorsed violent struggle as a means of overthrowing Marcos and instituting fundamental changes in the government. The communist-led military wing of the leftist opposition, called the New People's Army, operates mainly in the countryside and estimates of its strength vary from 5,000 to 10,000 members.

Although martial law, instituted in 1972, was lifted in 1981, Marcos retains what critics call authoritarian powers. His regime--supported by the U.S. government--has been criticized by such groups as Amnesty International for violating human rights, and currently the country is experiencing serious economic difficulties. The social and economic problems have bolstered the opposition movement, and the coming elections were viewed as a potential challenge to Marcos' party despite his broad powers.

The Rev. John Sebes, professor of Asian studies at Georgetown University, said the assassination "might endanger the future of Marcos' government." He called Aquino "the man who represented, in the eyes of the Filipinos, the resistance itself," adding that "the impact of this is incalculable."

An informed American observer of the Philippines said the assassination "leaves uncertainty in the opposition and uncertainty in the government." The foreign policy expert, who spoke on the condition that he not be named, said it was difficult to anticipate precisely what the opposition--or the government--will do in the near future. He said that much depends on "how the Marcos government handles the reaction." A crackdown on dissent, hinted at in a statement by Marcos after the killing, could lead to unpredictable results, he said.

However, some observers said the domestic political scene in the Philippines could be affected by international reaction. One opposition activist said, "The real big question is what the United States is going to do." Manglapus, the former senator, said the assassination "is an indication of how intolerable the situation has become." He and others expressed the hope that the United States and other countries would take note of rising troubles in the Philippines and ensure that fair elections are held and reforms instituted.

U.S. presidential candidate Walter Mondale called Aquino's death a "tragedy of major proportions," according to The Associated Press. The assassination, he said, "is bound to add further strength to the argument that the Marcos regime is oppressive, is a police state."

Rep. Stephen Solarz (D-N.Y.), who visited the Philippines last week, called the assassination a "dastardly deed," Reuter reported. Speaking in Bangkok, he said Aquino "represented one of the best hopes for a genuine democratic alternative in the Philippines."