Prime Minister Prem Tinsulanond added new turmoil to Thailand's muddled political scene today by announcing his retirement from government after three years in office.

The retired Army general told reporters he would not accept the premiership in a new government that is currently the subject of intense wrangling by major political parties. His unexpected announcement came a week after a parliamentary election that was hailed as a major step toward full democracy in coup-prone Thailand but which saw the vote fragmented among the leading parties.

Prem, 62, who belongs to no political faction and did not run for a parliamentary seat, was endorsed by the two major parties vying to form a coalition government and was considered a compromise candidate acceptable to the politically powerful armed forces. He was thus considered a shoo-in to head whatever new coalition emerged.

But the modest former Army commander apparently was alienated by fractious wrangling in the newly elected House of Representatives, pitting two major partners in his current three-party coalition government against each other in a bid for power.

Despite Prem's announcement, some western diplomats and Thai political observers said he might be persuaded to reconsider if a more stable coalition emerged. They tended to view his announcement as a rejection of efforts by the promilitary Chart Thai (Thai Nation) Party to form a coalition without the centrist Social Action Party led by eccentric former prime minister Kukrit Pramoj.

The Social Action Party captured the most seats in the 324-member house April 18, but Chart Thai persuaded a couple of smaller parties to merge with it after the election and now ranks as the largest party, with 108 seats to Social Action's 101.

King Phumiphol Adulyadej formally opened the new parliament yesterday, calling on legislators to cooperate for the stability and prosperity of the country. Prem later was reported to have turned down an offer to head a coalition formed by Chart Thai that excluded the Social Action Party and its allied Democratic Party. The three parties make up Prem's outgoing coalition.

"It's difficult to foresee much stability with the activist Kukrit and his Social Action Party plus the Democrats in opposition," one senior western diplomat said. Although the Chart Thai Party might be tempted to bank on the support of the Army, and the military-dominated, appointed Senate, to stabilize its coalition, that could be a "risky proposition" because of increasingly active students returning for a new term next month, he said.

For the time being, however, the political wrangling has shifted momentum back to the Army, which suffered a humiliating defeat in a parliamentary vote last month on a controversial bill to amend the constitution. The Army commander, Gen. Arthit Kamlang-ek, aggressively promoted the amendments aimed at maintaining the military's dominant role in Thai politics.

Regardless of the new government lineup, the Army now is considered likely to get its way in pushing the changes through the new parliament. The amendments would restore the full powers of the Senate and the eligibility of serving officers to hold political posts, both of which lapsed April 22 under provisions of the 1979 constitution.

Up to now, the military has been playing by the rules of Thailand's move toward coup-free democracy, but it has not stopped vigorously promoting its own interests.