A principal opponent of Philippine President Ferdinand Marcos was sentenced yesterday to be executed by a firing squad along with two Communist guerrilla leaders.

Former Sen. Benigno S. Aquino Jr., 44, who has been in jail since Marcos declared martial law five years ago, was convicted by a military tribunal in Manila of charges of subversion, murder and illegal possession of firearms.

Sentenced with him was Bernabe Buscayno, also known as "Commander Dante," a leader of the New People's Army, and a policeman turned guerrilla, Lt. Victor Corpus.

"If Marcos believes I'm guilty. I want to be shot tomorrow," Aquino said before being surrounded by security men and hustled out of the courtroom in Fort Bonifacio outside Manila.

THe decision was expected to spark protests both in the Philippines - where political activity and opposition to Marcos is severely restricted - and abroad.

The Marcos government has been widely criticized for violations of human rights under the strict martial law regime.

Congress cut military aid to the Philippines by $4 million this year because of reported human rights violations, and the Carter administration has urged the Philippines government to ease martial law restrictions.

In reaction to the death sentences, the State Department issued a terse statement saying: "We understand the next step is likely to be an appeal to the Philippines Supreme Court. We are confident that procedure will be followed prior to ultimate disposition of the case."

Some observers interpreted that statement as indicating some hope that the sentences would be commuted either by the Supreme Court or by Marcos, who is to review them.

Of the 150 death sentences imposed by military courts since martial law was decreed, only one was carried out. The condemned man was a narcotics dealer who was shot in January 1973.

Attorneys for Aquino and Buscayno said they will appeal on Saturday. Aquino has argued from the beginning of the trial that the seven-man military tribunal could not legally try him since its members owe their loyalty to Marcos, their commander-in-chief.

Buscayno, 34, said the sentences "will prove to be a politically fatal mistake for the Marcos regime."

Two other major political opponents of Marcos who had been imprisoned escaped to the United States in September. There has been speculation that the two, Sergio Osmena III and Eugenio Lopez Jr., were allowed to get away in order to ease human rights criticism without openly giving in to U.S. pressure.

Osmena, Lopez and Aquino all had confinement since he was arrested Sept. 23, 1972, first went on trial in 1973 after staging a 40-day hunger strike. The tral was later suspended and resumed in August of last year.

At the time of his arrest, Aquino was considered the most likely presidential candidate of the main opposition group, the Liberal Party, in elections scheduled for the following year.

The election was never held, although voters in two nationwide referendums since then have endorsed Marcos and martial law.STIn the trial, Aquino was charged with being a ranking member of the Communist Party, of financing subversives and organizaing violent anti-government demonstrations.

He was also accused of collaborating with Buscayno, who was arrested last year, to murder a village chief in his home province Tarlac.

Buscayno was convicted of murder and subversion.

Aquino's wife, mother and other members of his politically prominent family from central Luzon were in the courtroom when Brig. Gen. Jose Syjuco read the verdict and sentences at the close of a 12-hour court session.

Aquino kissed his six-year-old daughter, the youngest of five children, then kissed his wife Corry on the cheek and said: "It's all right. Don't you worry. That proves nothing more than the mockery of justice under martial law."