Turkish voters yesterday overwhelmingly endorsed a new constitution written by the military government that establishes a strong presidential system, curbs civil rights and limits the freedoms of trade unions, universities and the press.

With more than 75 percent of the votes in the referendum counted, the unofficial tally was more than 90 percent in favor of the constitution under which Gen. Kenan Evren, the country's military head of state, will become president for a seven-year term and 100 political opposition leaders will be barred from politics for 10 years.

The massive support for the 177-page document and its 16 "temporary articles" surpassed even the publicized expectations of the generals, who have been ruling Turkey by decree since they seized power in September 1980.

The wide popular backing for the new constitution was interpreted today by some experienced observers as an encouragement to Evren to expedite the restoration of democratic rule.

It does not indicate widespread satisfaction with his economic policies or the repressive military government, they cautioned.

In Washington, the State Department said, "We believe the constitution provides the basis for a democratic system of government and are confident that as the process of reestablishment of democracy proceeds, strong, viable, civilian political institutions will emerge."

Prior to the vote, some observers had maintained that the referendum would not be democratic because the country was under martial law and the government's opponents were not allowed to campaign.

The military did not allow anyone to challenge Evren's bid for the presidency, and the government's massive propaganda machine was mobilized to help him. State-controlled Turkish radio and television, on which the majority of the residents depend for their information, barred all opposition comment.

In addition, there were unconfirmed reports that the military was putting pressure on rural voters -- who account for 60 percent of the country's 45 million people -- to support the new charter.

But support for the constitution was surprisingly even -- as massive in the villages as in the cities where observers did not report instances of intimidation or pressure.

"Even if you calculate that 20 percent of the votes were fraudulent -- and I haven't heard anything to suggest that it was -- you are still left with more than 70 percent supporting the constitution , which is hefty enough," said a Western diplomat.

The constitution -- Turkey's third since the republic was declared in 1923--will go into effect after official vote results are published, probably Wednesday. However, Evren and the five-member National Security Council will continue to run the country with sweeping powers until general elections, which are scheduled to take place in the fall of 1983 or the spring of 1984, are held.

Politics will remain banned until a new law on political parties is written. Evren will be president, chief of staff and chairman of the National Security Council. After the elections the four other generals in the council will form the Presidential Council and continue to help Evren rule.

The previous two-house parliament has been replaced by a single chamber of 400 members. Under the new charter, the Cabinet will have limited powers.

The president's powers, on the other hand, have been strengthened. He is empowered to dissolve parliament and submit constitutional amendments to referendum. He will appoint the prime minister and "accept his resignation." He will be able to call Cabinet meetings "whenever he deems necessary."

The authority for declaring a state of emergency or martial law and ruling by decree will belong to the Cabinet sitting under the president. The president is also empowered to make senior appointments in the Army.

Among the political leaders barred from politics for 10 years under the new charter are former premiers Suleyman Demirel and Bulent Ecevit, both of whom made no comment yesterday. The constitution received 92 percent "yes" votes in their former constituencies.

The banning of the leaders of the now-abolished parties means that new political parties will have to be formed once the new parties law is passed and that new leaders will have to be found for them.

It is not yet known how the military leadership will interpret its success in the referendum. Some analysts suggest that Evren will now back a new political party to create a power base for himself in the new parliament.

Some observers also suggest that he might adopt a tougher stance against the left, while pressing ahead with the economic austerity program prescribed by the International Monetary Fund.