In almost eight years of imprisonment that included threats of a death sentence, hunger strikes, impassioned appeals for an end to martial law in the Philippines and finally the two heart attacks that brought about his release, Benigno Aquino remained a thorn in the side of Philippine President Marcos.

Now, the once flamboyant senator, who was the betting choice to defeat Marcos had elections been held as scheduled in 1973, is adding impetus to the exiles' anti-Marcos movement in the United States with a series of speeches.

The message he carries to U.S. audiences, but aims at Marcos, is that the president must relax martial law and gradually return the Philippines to democracy or there will be violence.

"There is a gathering storm," he told a National Press Club breakfast yesterday.

Threats of violence have been the stock in trade of Filipino politicians of all parties for years. Aquino, however, claims to have surveyed opposition groups, including the Moslem insurgents of the southern Philippines, and found they share a belief that time is running short for a peaceful settlement of the island nation's political differences.

He cited the current trial to Eduardo Olaguer, whom Philippine authorities say was responsible for several bombings in the Manila area as head of an urban terrorist group called the "Light the Fire Movement."

Opposition groups, with the exception of the Communist Party, have signed a National Concord for Freedom, Aquino said, demanding the lifting of martial law which Marcos declared in September 1972, an independent foreign policy, and a restructured, more democratic society.

Although the several opposition groups have had difficulty working together in the past, Aquino said that they plan to cooperate under a collective leadership in which "there will be no boss."

The Communists are not included in the opposition coalition because they demand an anti-American policy and blame "American colonialism" for Marcos, Aquino said. "Our problem is with Marcos," he said. "This is an internal Philippine problem."

He said he favors keeping American bases in the Philippines. "We prefer the devil we know to the devil we don't know."

He considers himself a personal beneficiary of the Carter administration's human rights policy. After Carter proclaimed his concern for human rights, the treatment of prisoners in the Philippines improved, he said. Torture was used less frequently and more prisoners were released, he said.

Aquino was given permission to fly to Houston for open heart surgery in May. Since then, he said, Marcos has extended indefinitely his permission for Aquino to remain in the United States.

"You want to see my scar?" Aquino asked two friends before the breakfast began. Before they could answer, he opened a button of his elegant blue and white striped shirt to show the long scar down his breastbone.

Before leaving his country, Aquino signed a promise to return. He also pledged to refrain from political activity and -- at Marcos' request -- to leave at least one of his children in the Philippines as a guarantee of his good behavior.

At the time that he signed, Aquino said, "I was very weak" after the second heart attack.

He said he still intends to return after completing a fellowship project at the Harvard University Center for International Affairs, but he has violated the other two conditions of his release.

His Harvard task is to draw up a model of how a country could move peacefully from one-man rule to democracy.