A former Green Beret, charged with attempting to murder a leader of Libyan dissident students, is on trial here in the first public examination of evidence suggesting that Libyan leader Muammar Qaddafi, with the help of American former Special Forces and ex-CIA agents, was attempting to conduct an assassination program in this country.

The prosecution in the trial, which is selecting jurors this week, alleges that the ex-Green Beret, Eugene Aloys Tafoya, 47, of Truth or Consequences, N.M., shot Faisal Abdullizez Zagallai, a 35-year-old graduate student at Colorado State University, here on Oct. 14, 1980.

In affidavits filed in Larimer County District Court, police say Tafoya, a retired veteran of 23 years in the U.S. Army and once a member of the elite Special Forces, is a "mercenary soldier who is available to be hired and paid to kill human beings." He also is described by police in the affidavits as a "hit man" hired by Edwin P. Wilson, a former agent of the Central Intelligence Agency who has been working for the Libyan government since 1976.

Wilson and another ex-CIA agent, Francis E. Terpil, were indicted in April, 1980, in Washington on charges of acting as foreign agents for Libya, of shipping explosives and other materiel and in connection with a conspiracy to murder a Qaddafi critic in Egypt. Wilson is a fugitive in Libya; Terpil is in Beirut.

In a hearing here last week, police investigators alleged that Tafoya was in contact with James Clinton Dean, 51, of Fayetteville, N.C., another former member of the Special Forces. Prosecutors have attempted to contact Dean and call him as a witness. Reporters' attempts to reach him have been unsuccessful. Dean has not been charged with any crime.

On Oct. 14, 1980, Zagallai was shot twice in the head. Zagallai told police the man who shot him had posed as a recruiter for a company doing business in the near East. Three days later, the official Libyan news agency claimed responsibility for the attack.

A trail of credit card records, motel receipts, telephone calls, bank deposits and passport entries led investigators to Tafoya, who was arrested in New Mexico last April.

In a search of Tafoya's home, police claim to have found a "hit list" of names, addresses and habits of people who had "a falling out with Edwin P. Wilson," a deputy district attorney said in pretrial hearings.

Police claim to have found a cassette tape in Tafoya's car. On the tape, according to a transcript introduced at pretrial hearings last week, a man identified by police as Tafoya asked, "Do you know anyone who should stop breathing--permanently? "

Another voice, alleged in police reports to be that of Dean, replied, "Okay. Give me a target."

In the conversation, the man identified as Tafoya takes credit for the 1979 firebombing of a car belonging to a Canadian businessman once associated with Wilson.

Tafoya insists he was working for the CIA at the time of the Zagallai shooting. The CIA denies this.

Tafoya's lawyers say they will attempt to portray Zagallai as a pro-Qaddafi agent who infiltrated anti-Qaddafi groups to inform the Libyan regime about dissident activity.

According to an affidavit by Fort Collins police detective Ray Martinez, Tafoya said that his mission in visiting Zagallai was merely "to slap him around a little bit" so Zagallai would "pay attention" when he received a phone call the next morning. Tafoya said his purpose was to "deliver a message" from the CIA warning Zagallai to stop his activities in support of the Palestine Liberation Organization, Martinez said in his affidavit.

Tafoya claimed that Zagallai pulled a gun on him first, and that only then--in self defense--did he shoot Zagallai, Martinez said Tafoya told him.

Tafoya refused to name the CIA agents for whom he supposedly was working. But Martinez contended in his affidavit that Tafoya described himself as an "associate" of Wilson and said he knows Terpil.

At one point in their conversation, Martinez said in the affidavit, "Tafoya started crying and expressed his disappointment that the U.S. government didn't back him in the Zagallai matter."

Martinez also said in his affidavit that Tafoya had said he was afraid he would "get his throat cut" if Martinez revealed the contents of their conversation. Last May 7, according to a police report, Martinez told police in Albuquerque, where Tafoya was being held, that "we have received reliable intelligence information that an attempt to assassinate Eugene Aloys Tafoya . . . is going to be made prior to his extradition to Colorado."

Police have placed extraordinary security measures at the local courthouse for the trial. Tafoya's attorney, Walter Gerash of Denver, describes the scene as "an armed camp." He also has said his client is reluctant to testify in his own defense out of "fear for his life."

The precise nature of Tafoya's "business" is under wraps until testimony begins later this week. A number of CIA agents from Washington are expected to testify that Tafoya wasn't working for the CIA--at least not when he allegedly shot Zagallai last October.