An elderly Vietnamese couple found slain in their Northwest Washington home Thursday feared attack by the 60-year-old son they were trying to evict and repeatedly expressed their concerns to a relative shortly before they were beaten and suffocated, according to D.C. police.

The couple, Tran Van Chuong, 88, and his wife Nam-Tran Chuong 75, both former diplomats here, had argued with their son the night before their bodies were discovered over their request that he move out, according to a police affidavit filed yesterday in D.C. Superior Court.

The son, Tran Van Khiem, charged with first-degree murder in his parents' death, was being held yesterday in lieu of $100,000 bond and was ordered to surrender his French passport and visa after telling a Superior Court judge that he intends to commit suicide because he believes he is innocent.

Khiem, an attorney in Paris and former paramilitary official in South Vietnam, told Judge Richard S. Salzman that he mailed President Reagan a letter a half-hour before his parents' bodies were found in their bedroom and that "President Reagan can testify that I am innocent."

A White House spokesman had no comment on Khiem's statement.

An attorney for Khiem, Thomas A. Mauro, said Khiem has told him "several times" that he intends to take his own life.

"He feels that he's innocent," Mauro said. "The evidence is circumstantial. He thinks this entire proceding against him is unfair."

Tran Van Chuong was a South Vietnamese ambassador here, and Nam-Tran Chuong was South Vietnam's permanent observer to the United Nations until they resigned their posts in 1963 to protest the treatment of Buddhists under the regime of President Ngo Dinh Diem.

Their pajama-clad bodies were found by a housekeeper around 10:50 a.m. Thursday in their home at 5601 Western Ave.

Both suffered "multiple blows to the face," according to the police affidavit, and they were asphyxiated.

Their son Khiem, who was living in the house along with his wife and 12-year-old son, said he had come to Washington from Paris after receiving a call for help from his mother. He told The Washington Post before his arrest that the family had been leading a "quiet" life together and that he believed that his parents died of natural causes.

The affidavit filed by police, however, portrayed a family torn by frequent arguing over Khiem's refusal to leave. Police said some of the evidence leading to Khiem's arrest came from his wife and son, who, according to the affadavit, contradicted Khiem's version of events the night his parents were killed.

According to the affadavit, the slain couple had "repeatedly argued" with their son in recent weeks and again Wednesday night quarreled with him over their "repeated demands" that Khiem leave with his family and return to Paris.

Khiem's son went to bed around 8 that night and Khiem's wife Mirielle followed him to a third-floor bedroom around 10:30 p.m., according to the affidavit. Around the same time, police said, Nam-Tran Chuong began a series of telephone calls to a daughter in North Carolina saying she and her husband "were fearful that Khiem would hurt them."

The couple may have been killed as early as 11 p.m., according to police. The next morning around 10:30, when the couple had not come downstairs for breakfast, Khiem "told the housekeeper to accompany him to his parents' room" where they found the bodies, the wife's sprawled across the husband's.

During questioning Thursday, police said, they "observed some discoloration on Khiem's knuckles consistent with bruising" and a small scratch on his left arm.

Khiem told the Post on Friday that homicide investigators had accused him of using karate against his parents, which he denied.

Saltzman ordered that a "suicide alert" be enforced at the D.C. Jail, where Khiem is confined. Khiem is scheduled to appear in court Aug. 7 for a preliminary hearing on the murder charges, before the case is sent to a grand jury for possible indictment.