The Vietnamese ambassador to the United Nations left the United States last night for Paris despite a court order directed at keeping him in the country to testify in an espionage trial.

Ambassador Dinh Ba Thi said his government had instructed him to leave the country, and called allegations that he was part of an espionage conspiracy a "slanderous fabrication."

Thi, who has diplomatic immunity, was "requested" by the State Department to leave the United States after he was named an unindicted coconspirator in a federal espionage case. He refused, and vowed to remain at his post.

Two days later, Thi's government recalled him, but lawyers for Ronald Humphrey, one of two men charged with spying on behalf of Hanoi, went to court to keep the ambassador in the country.

U.S. District Court Judge Albert V. Bryan Jr. yesterday prohibited the U.S. from trying to force the ambassador to leave.

As Thi and his wife boarded an Air France plane for an economy flight to Paris last night, an aide said he did not know if the ambassador, was aware of the judge's order.

Lawyers for Humphrey who is accused of stealing classified diplomatic cables for an international network of Vietnamese communist agents, argued yesterday that Humphrey needed Thi in the U.S. because he is an important witness in the case.

But Thi told reporters yesterday at a news conference at the United Nations that his country had instructed him to leave "because the U.S. government has grossly obstructed my normal activities in the performance of my duty as representative to the United Nations.

"The recent U.S. administration's slanderous fabrication toward me constitutes an unprecedented and serious event in the history of the U.N., jeopardizing the normal activities of member countries and of the organization of the U.N.," Thi said.

"I firmly and definitely state that the U.S. government cannot give any legal evidence whatsoever to acuse me and energetically protest the U.S. government for its slanderous allegation about me," Thi continued. "This action will affect the normalization of relations between Vietnam and the U.S."

At Kennedy Airport, Thi said, "If I have an opportunity, I will be back. According to my government's instructions."

Thi spoke in Vietnamese and answered questions through an interpreter. "We're very proud of our work here. Justice will win," he said.

Humphrey's lawyer, A. Andrew Giangreco, asked the court to keep Thi in the country because "to permit this witness to be ordered out of the country will deprive (Humphrey) from the benefit of competent and material and relevant testimony and constitutes a clear violation of his constitutional rights."

But because Thi is a diplomat, the order did not apply directly to the envoy, but only prohibited the United States from forcing him out of the country.

Bryan told Humphrey's lawyers in court that his order could not prevent Thi from leaving the country because he has diplomatic immunity.

Yesterday's events began with a court hearing before Bryan at 10 a.m., when the judge handed down the temporary restraining order, prohibiting the United States from attempting to "procure, encourage, obtain or request the expulsion from" the United States of the ambassador.

By 4 p.m. Thi had announced his departure, and three hours later the ambassador denounced the United States and boarded a plane for France. Meanwhile, Humphrey's attorneys, who were visiting their client in the Alexandria jail, were unaware that the ambassador had left the country.

Air France said the U.S. government had not contacted the airline about Thi's departure.

The Vietnamese ambassador is the first U.N. ambassador to be "requested" to leave.

Humphrey's attorney said he would be willing to have Humphrey testify as to how Thi would affect his case, but the judge said that was not necessary. Giangreco told a reporter after the hearing that Humphrey would have testified "that he never met the man (Thi). He wouldn't know him if he walked in this room."

Truong's attorney, Michael Tigar, said that last Friday he spoke to the attorney for the Socialist Republic of Vietnam and a member of Vietnam's U.N. mission. "They indicated to me at that time that the ambassador is in possession of information that would . . . aid my client."

But the counsel for Vietnam was unable to assure him that Thi would cooperate with them, Tigar said.

Yesterday Bryan denied a request by Truong's attorneys to reduce his bond from $250,000. Truong's attorneys presented affidavits from Truong's friends, including former U.S. Attorney General Ramsey Clark, attesting to the unlikehood that Truong would flee if released on a lower bond. Humphrey is being held on $150,000 bond. Both men are in Alexandria jail.

Clark said, in his affidavit, that he has known Truong seven years, primarily through the Vietnam war peace movement. "I found him to be an extremely sensitive and compassionate human being who suffered greatly throughout the Vietnam war. His veracity, character, reliability and sense of responsibility are of the highest quality. I have found him to be dependable and mature."

Last night Truong's attorneys filed an appeal of Bryan's decision to the Fourth Circuit Court of Appeals.