The Turkish government today ordered more than 200,000 troops and policemen to maintain order Sunday during parliamentary by-elections expected to usher in a prolonged political crisis here.

The voting for five of the National Assembly's 450 seats follows Prime Minister Bulent Ecevit's coalition government's loss of a parliamentary majority because of deaths and defections in recent months. He has remained in powder because of a long parliamentary recess.

Political observers here expect Ecevit's Social Democrates to lose all five assembly contests as well as the majority of the 50 Senate seats at stake, or a third of the upper chamber.

Former prime minister Suleyman Demirel, leader of the conserative opposition, will attempt to topple the Ecevit government when the parliament reconvenes on Nov. 1. It is by no means certain that Demirel would be able to form a viable government, however.

The prospect of a prolonged political crisis comes at a time when Turkey is involved in negotiations about the future of U.S. military facilities on Turkish soil. Moreover, the country is facing a severe economic crisis.

Many political observers here fear that Turkey's military chiefs may decide to disband parliament if the politicians fail to produce a government capable of resolving the country's mounting internal difficulties.

Three include the question of law and order. More than 2,000 people have lost their lives in the last two years in clashes between extremists of the right and left who are fueling racial and sectarian riots among Turks and the Kurdish minority, and the orthodox Moslems and the minority Alawites.

Inflation is running at more than 60 percent a year, unemployment is well above 25 percent among the 45 million, predominantly farming population and the foreign currency reserves of the central bank are almost depleted.

The economic crisis is continuing despite the restructuring of the huge Trukish foreign debt and foreign debt and commitments of new credits by Western states and banks.

Ecevit's popularity has waned because of his liability to come to grips with the violence and the economic crisis.

If Turkey relapses into a prolonged crisis after Sunday's elections, most observers fear the military will intervene at some time, despite its wish to stay out of the mess.

Twice in the past 19 years Turkey's bickering politicians have forced a reluctant military to intervene. They seem likely to do so again.