A House committee yesterday began the first full-scale open investigation of the assassination of the Rev. Martin Luther King Jr. with a moving recollection of King's last months.

In a deep voice filled with sadness, the Rev. Ralph David Abernathy, King's closest associate and his successor as head of the Southern Christian Leadership Conference, told the Select Committee on Assassinations that King seemed tortured by a premonition of death for months before the day in Memphis when he was felled by a single shot from a high-powered rifle.

"This is just my belief," Abernathy said, "I cannot support it. But I think he had received some word . . . that he was going to be assassinated."

From about October 1967 to King's death on April 4, 1968, Abernathy said, "Dr. King was a completely different individual. He was troubled.He was worried. He was nervous, and very, very jittery."

Abernathy reiterated his belief that King's killing was the product of a conspiracy, and said the killer probably had help from the Memphis police department and possibly from the FBI.

Abernathy said this belief, too, was based more on intuition than on substantive evidence.

Explaining his view of the crime to reporters after the hearing, Abernathy said he believed powerful figures were concerned when King expanded his campaign for racial equality into a drive to end the war in Vietnam and redistribute the nation's wealth.

"I think when Dr. King started to bring poor people together," Abernathy said, "someone in a very powerful position decided that we must eliminate this young black leader, we must cut him down.

Abernathy said he had recently received a letter from James Earl Ray, the itinerant robber who confessed to killing King but has since recanted his confession. Abernathy said he was asked in the letter to help Ray win a new trial on the murder charge.

He said the letter promised that, if granted a new trial, Ray would reveal who financed his flight to Canada and Europe after the King slaying.

Ray is scheduled to testify publicly before the committee tomorrow, but it is not clear whether he will be a cooperative witness. His lawyer, Mark Lane, charged after yesterday's hearing that the committee seems predisposed to blame the King murder on Ray alone.

Yesterday's session marked the first public fruits of the 18-month, $5 million congressional investigation of the assassinations of King and President Kennedy. Hearings on the King case will go on this week and for three weeks on November. The Kennedy assassination hearings are set for next month.

Fearful of adverse publicity that might undermine the credibility of its work, the committee hewed closely by its staff. Committee members read their remarks from a prepared script that told them when to talk and when to yield to colleagues.

Under the grand design of the committee counsel G. Robert Blakey, yesterday's opening session was supposed to "introduce the major character and the death scene," Abernathy, the day's only witness, did that in a poignant memoir of his work with King in the civil rights movement and on their last trip to Memphis.

Abernathy, 52, who was best known in the 1960s as a tough, overalls-clad activist shouting his demands at the white establishment, had the air yesterday of a distinguished, slightly graying elder statesman of the movement.

His recitation of King's efforts to aid Memphis striking garbagemen covered ground that the committee and its staff seemed to know well. But he brought the session to a hush when he described the moment of King's death on the balcony of the Lorraine Motel.

"We put on our coats and set out from the motel room," Abernathy said slowly." . . . I heard what sounded like a firecracker. I jumped, and looked out at the balcony.

"He had fallen directly back. He had fallen into the room. The bullet was so powerful it had twisted his body, and he fell in sort of a kitty-corner fashion.

"The bullet had entered his right cheek. I patted his left cheek . . . He didn't say anything. His mouth quivered only once. But he did look me dead in the eye.

"I . . . heard the groans of the people in the courtyard below."

Abernathy testified in matter-of-fact tones about the FBI's long harassment of King and his followers.

"Our telephone lines were tapped and we knew they were tapped," he said. "We learned to live with it . . . The FBI saw us an enemy because we were seeking to bring about change."

Abernathy said the FBI provided no security for King on his last Memphis trip, although the bureau knew of threats against his life. He said the FBI recruited the young blacks who provoked violence at King's last civil rights march, in Memphis a few days before his death. He said that was an effort to tarnish King's image as a nonviolent leader.

The witness, who was King's closest associate for years, told the committee that none of the law enforcement officials who investigated the King murder ever talked to him. The FBI said yesterday it had interviewed Abnernathy. The bureau did not say where or when.

Abernathy returned several times to the discussion of King's depressed state during his last months. He said King was "frightened" at times, and gave explicit instructions for the sermon to be delivered at his funeral.

In October 1967, Abernathy recalled, the two friends took a vacation in Acapulco to soothe King's troubled spirit. But King could not sleep, and stayed up into the night staring off at the sea.

"Once I got u around 3 a.m.," Abernathy said. " . . . I said, 'Martin, what's bothering you? What are you thinking?"

"He said,'you see that rock out there? . . . Don't you know what I've been thinking?' And then he started singing, very softly. 'Rock of ages, cleft for me, let me hide myself in thee.'"

"This was," Abernathy said painfully, "this was my dearest friend. The nights have been long since he went away."