Eighteen Thai students charged with treason, murder, Communist agitation and a variety of other major crimes pleaded innocent to all counts before an unusual open trial by a military court today.

The students have been imprisoned since Oct. 6 last year, when a bloody gun battle between students and police at Bangkok's Thammasat University gave rise to a military coup and the fall of the frail democratic government of former Prime Minister Seni Pramoj.

The military-backed regime of Prime Minister Thanin Kraivichien took the rare step of opening the martial-law trail to the public presumably because U.S. and other Western officials as well as many.Thais expressed concern that the students receive a fair trial.

Some prominent Thais and private Americans resident here have appealed to the regime to insure a fair trial and to allow the students to have lawyers. Their lawyers have been barred from participation in the trial on the grounds that legal aid is banned in military courts.

The three-member military court, headed by Air Vice Marshal Sansern Wanich, charged six of the 18 with lese majeste. This stemmed from a mock hanging staged on the university campus the night before the battle with police. The hanging "victim" was allegedly Crown Prince Vachiralongkorn.

The murder charges against all 18 were based on the gunshot deaths of two policemen during the campus fight. According to official figures, 39 students were also killed Pravoon Akkarabovorn, one of those charged today, told journalists covering the hearing that the student death toll was "over 200."

Although the U.S. embassy has made no comment on the trial, a staff member attended the hearing as did a legal expert from the joint U.S. Military Assistance Group. Don Luce, a former anti-Vietnam War activist and author, now a member of the New York-based Concerned Laity and Clergy, also sat in.

Last week, U.S. Supreme Court Justice Thurgood Marshall met in Bangkok with Premier Thanin and, according to informed sources, they discussed the case.

The student have been granted two-20-minute meetings a week in their cells with lawyers. But they have not been allowed to see any documents regarding the case or to have the lawyers accompany them to the court.

Sutham Saengprathum, former secretary general of the banned National Student Council of Thailand, told the three-man tribunal that the students are entitled to legal counsel and that the military court does not have the right to try them.

He said that the crimes with which they were charged were committed before the imposition of martial law. According to the charges, some of the students allegedly carried out treasonous acts as far back as 1974. A student-led demonstration on Oct. 14, 1974, led to the overthrow of the previous military regime and the start of Thailand's tenuous experiment with democracy.

More than a hundred relatives, several of them weeping quietly, and reporters and other observers packed the tiny courtroom at Defense Ministry headquarters. Another 2,000 people stood outside the building, listening to the proceedings over loudspeakers.

The student's appeared healthy and said they'd been well care for after an intial six or seven months of harsh treatment.

The trial is scheduled to resume Sept. 7 and 13. The government has prepared some 200 witnesses. The students are not expected to be allowed to call any witnesses for their defense.