President Ferdinand Marcos has decided to declare an end to eight years of martial law and may hold new elections shortly as part of an effort to quell growing doubts abroad about the stability of his authoritarian rule.

The decision to lift martial law in the Philippines will be announced within the next 90 days "unless the situation drastically changes" because of something as catastrophic as an invasion or an economic collapse. Marcos said in an interview with The Washington Post, "I don't expect any of these things to happen," he added.

This represented a significant reduction of conditions Marcos previously placed on ending martial law, which he has used to silence critics and to extend his own term of office as well as to suppress armed uprisings.

But Marcos suggested that he would leave in place many of the restrictive laws that give him near dictatorial powers even after martial law formally ends. "It is possible that the phasing out will be gradual," he said.

Marcos is moving to spruce up an image that has become badly tarnished in recent years just before two events that will have particular impact in Manila. They are the change of administrations in Washington in January and the planned visit here in mid-February of Pope John Paul II.

Marcos, who was a target of human rights activists in the Carter administration, clearly feels he is better positioned to work with the incoming Reagan government, and he said he hopes the recent "irritations" between Washington and Manila over human rights will disappear now.

The 63-year-old leader dealt tolerantly with questions for about an hour in his spacious office at the Malacanan Palace. Seated behind an imposing desk positioned on a raised platform, he smiled down frequently at his questioner and gave no signs that he feels as beleaguered as political opponents, some Carter administration officials and worried international bankers have recently portrayed him.

At the same time, his statements illustrated Marcos' proven ability over time to diffuse internal pressure by promises of change and hopes for the future. Such maneuvers infuriate his opponents and traditionally have left them powerless.

These critics maintain that Marcos, his family and his close associates in government head a regime discredited by corruption and persecution of opponents. Citing Iran, they maintain that American interests, which include vital naval and air basing rights here, will ultimately be harmed by U.S. support for Marcos.

Marcos loftily dismisses such charges as "distorted" and asserts the airing of them by the press and by "some of the officials the United States sent here, has created doubts about his ability to continue to rule this nation of 50 million people.

Later in the interview, he acknowledged that his questioning has led him to reconsider the image his government has abroad and to make the first moves toward dismantling martial law in the near future.

The continuation of martial law appeared to be "the principal cause of all of this misunderstanding," he said. "Americans not trained in the fine subtlety of the laws" here equated the phrase martial law with total and arbitrary military rule, he said, whereas his government was a civilian one that used the military to carry out its policies.

Noting that he had said in September 1979 that he hoped to end martial law in 18 months, Marcos reaffirmed that "March is our deadline." On initially setting that deadline, however, Marcos had given himself wide leeway for reversing the decision.

Marcos said he has not taken any position yet on the possibility of a presidential election next summer if martial law is lifted, although that has received much attention in the government-influenced Manila press this week.

"If the people desire an election, I will comply with their wishes," he said, adding that on a recent tour of flood-ravaged regions, "I found people are preoccupied not with politics but with economic rehabilitation."

After the lifting of martial law, military tribunals already hearing cases involving subversion or sedition would be phased out gradually, Marcos said. In a separate interview, his defense minister, Juan Ponce Enrile, said the government intends to keep the power to suspend the right of habear corpus in selected national security cases.

Enrile estimated that 300 to 500 persons now in detention were awaiting trial by the military tribunals on national security charges. As many as 200 of those prisoners are dedicated communists not eligible for amnesty, he added.

Marcos asserted that his government's investigations had turned up only four verifiable cases of torture in eight years of martial law and had investigated 100 cases of other alleged abuses.

"That is all there has been. This has been blown up into something hideous and monstrous" by human rights activists, he said, adding that Amnesty International, the International Committee of the Red Cross and other groups were welcome to visit Philippine prisons and see any prisoners.

Throughout the interview, Marcos alternately presented two basic assessments: in respone to one question, he said life was rapidly returning to normal here because of the strong leadership and firm hand he had provided; then, responding to a question about the arrest of a university magazine editor last week, he said the threat of terrorism and sedition were still too grave to permit reintroduction of the kind of legal protection built into the American system.

Marcos implictly acknowledged that a disproportionate number of the philippines' best-educated and most affluent youths seemed to be involved in terrorist groups but he dismissed this phenomenon, saying it was merely "fashionable for youth to be involved in agitation. They will grow up and learn. American youths were agitating when it was fashionable, and now they seem to have mellowed."

The president also indicated that if elections were held next summer, Benigno Aquino, the best-known opposition leader, would not be permitted to run. Last spring, Aquino won temporary freedom from prison, where he spent eight years after being convicted on a murder charge by a military tribunal. Marcos said "convicted terrorists" could not run for office.

Aquino is currently studying at Harvard on a fellowship, and was accused by Marcos last month of having been involved in a U.S.-based bombing campaign aimed at Marcos.

Some diplomatic analysts predict that Marcos' offer of a fair presidential election could split the already badly divided opposition even further. Asked if the opposition would participate, Gerardo Roxas, one of the opposition's leading figures, said Marcos' opponents are "extremely skeptical that there will be free and fair elections. Our skepticism is based on this government's previous record, and the powers it intends to keep even if it removes the name martial law."