Thailand's Parliament today defeated a controversial bill that would have preserved the Army's key role in politics, dealing a sharp setback to the powerful Army commander-in-chief, Gen. Arthit Kamlang-ek, who had been emerging as the country's strongman.

The defeat immediately sparked one of Bangkok's periodic bouts of coup jitters, but the Army generally is seen as being in no position to act right away. Instead, Thai and foreign political observers said, the Army more likely will have to wait for a better opportunity if it wants to make up for today's political loss.

The showdown between factions of Thailand's civilian politicians and military commanders amounts to the country's biggest government crisis since an April 1, 1981, Army coup attempt against the administration of Prime Minister Prem Tinsulanond, himself an Army general.

Arthit, then deputy commander of Thailand's 2nd Army Region, was instrumental in putting down what came to be called the April Fool's coup. He then began a meteoric rise in the Army hierarchy, becoming commander-in-chief last October and growing increasingly visible and outspoken on political issues.

In January, Arthit threw the country's political scene into a turmoil when he called for amendments to the 1979 constitution, effectively nullifying three provisions due to take effect in April during the runup to scheduled June 12 general elections.

The provisions in question would have reduced the power of the 225-member appointed senate to largely ceremonial duties; would prohibit civil servants, including military officers, from holding political positions such as cabinet posts; and would introduce a new voting system for the 301-member house. The system would turn each province into a single constituency and replace the multiple constituencies that tended to favor smaller political parties.

"Arthit is seen as very ambitious, and these clauses stood in the way of the easy exercise of his ambitions," a western diplomat said. "If the power of the senate were reduced, it would be hard for the Army to dominate legislation, especially the budget."

In addition, Arthit is known to covet the defense minister's portfolio, currently held by Prem. His ultimate ambition, it is widely believed, is to replace Prem eventually as prime minister.

Although the amendments sailed through parliament easily on the first two of three readings, opposition began to mount, and a key party changed its position before the crucial third reading today.

Student opponents called the changes undemocratic. A small group of hunger-strikers including a 10-year-old boy began holding a vigil near the parliament. A former political activist and police major quit the Buddhist monkhood to lobby against the amendments, and one legislator threatened to commit suicide if they were passed.

For its part, the Army issued two white papers supporting the changes, as well as some veiled threats to promote their passage. One general close to Arthit said the Army would persevere with the amendments until it won, and another warned that the military would "stage an exercise" if security considerations demanded.

Rightist groups including the Red Gaurs joined the fray, reviving fears of the bloody right-left clashes that marred Thai politics in the mid-1970s.

Rumors of an imminent coup if the amendments were rejected began circulating, but these were generally seen as an effort to influence today's vote.

"In Bangkok, rumors are not only an art but a tool," one western diplomat said.

Prime Minister Prem tried to stay out of the tussle, appealing to all parties to show restraint, avoid violence and accept whatever the parliament decided.

Nevertheless, opponents and supporters of the bill nearly came to blows last night when a group of soldiers appeared at an opposition rally and addressed the crowd through loudspeakers. Opponents chased away the leader of the group, Col. Jaruay Nimdits, by throwing shoes at him.

In today's vote, the amendment bill was defeated when its supporters could muster only 254 votes, 10 short of the 264 needed for passage. A separate bill calling only for a change in the electoral system also was defeated.

After the vote, about 1,000 opponents of the amendments dispersed peacefully, observed by about 400 police. After hearing the results, one member of parliament, Chalard Vorachat, who had vowed to kill himself if, as previously expected, the amendments passed, broke down in tears and fainted.

Ironically, observers pointed out, Arthit contributed to his defeat by resigning from the senate in what he said was an act of selflessness when he called for the amendments in January. He was joined by 10 other Army officers--the margin of today's loss.

"Arthit's own personal credibility will be hurt a lot, but he'll get another chance" because of the likelihood that the new political process will engender some party bickering and instability, a western diplomat said. "The question is if the Army will encourage a pretext for action."

A leading opponent of the amendments, the tart-tongued Social Action Party leader and former prime minister Kukrit Pramoj, recently confronted the coup question head on. If the Army were going to stage a coup, he said, it should do it before the June elections so that the country would not waste time and money holding them.