philippine President Ferdinand Marcos' leading political opponent, former Sen. Benigno Acquino, said yesterday that he plans to run for a seat in the new National Assembly while confined to his jail cell.

The decision by the country's most prominent political prisoner focuses an international spotlight on an election that has great bearing on U.S.-Philippine relations.

Aquino, 45, is expected to turn what might have otherwise been another routine pro-Marcos referendum into an emotional, well-publicized debate over the strengths and failings of Marcos' martial law rule.

Aquino's sister, film director Lupita Aquino Concio, disclosed his plans after a visit to his book-lined cell at Fort Bonafaci in Manila.

"I'm definitely funning. I'm going to file my papers for candidancy," she quoted Aquino as saying.

Aquino was jailed at the beginning of martial law in September 1972 when he was the acknowledged front-runner for what would have been the 1973 presidential election. Marcos' militarly aides charged Aquino with subversion, murder and illegal possession of firearms but failed to present a persuasive case.

When the military tribunal suddenly sentenced him to death three months ago, there was an international outcry, including an expression of concern from a U.S. State Department spokesman. Marcos quickly set the verdict aside saying he wanted to allow Aquino to present more evidence.

Despite allthe political headaches for Marcos in an Aquino candidancy, it would help the Philippe leader in one way by lending a measure of respectability to the April 7 election of a new, and essentially powerless, interim National Assembly.

Many Aquino advisers argued that he should boycott the election as a fraud, since Marcos will have the power to dissolve the new assembly at will. Aquino had several meetings in his cell with friends and family on the matter.

Yesterday's statement was the first indication that he had decided that the need to publicize his case against martial law outweighted his doubts about the election's honesty and the assembly's power.

Aquino's wife, Corazon, called a press conference for today in which she is expected to formally announce his candidancy. SHe is from a politically active and still wealthy family and is expected to be an effective campaigner on his behalf.

The Philippine National Security Council ruled Tuesday that persons detained for martial law offenses would be allowed to run in the election, but appeared to rule out their temporary release from prison to campaign. They would not be entitled to preferential treatment merely because of such filing," said Informational Secretary Francisco Tatad.

Marcos told reporters last week that nothing prevented Aquino from running if he wished. But some political observers still doubt that a political as astute as Marcos will allow his chief critic such a forum for national and international publicity. Some suggested that the certificate of candidancy, which Marcos pointed out Aquino would have to sign, might pose a stumbling block since if includes allegiance to the constitution that allowed Marcos to assume martial law powers.

One key Marcos adviser said that, given the Carter administration's human rights campaign and Manila's desire to get a new Philippine bases treaty approved by the U.S. Congress, Marcos had to make this first national election since martial law look as authentic as possible.

"The elections are being held at the suggestion of the U.S. State Department and to present to the U.S. Congress a martial law regime that looks more democratic," the adviser said.

Marcos is seeking stronger Philippe controls over two huge U.S. bases and a doubling of U.S. payments to about $200 million a year.