Greek Orthodox Americans have begun a nationwide campaign charging that the ecumenical patriarchate in Istanbul, Turkey, is being denied rights by the Turkish government and that its existence is under threat by the government.

Representatives of the Turkish government deny the charge.

The ecumenical patriarchate is the governing seat of the Greek Orthodox Church and the historic spiritual center of worldwide Orthodox Christianity, whose membership in 15 different ethnic branches is estimated at more than 100 million.

The roots of the Orthodox churches go back to the first century, when St. Andrew is believed to have established Christianity in Byzantium and other parts of Asia Minor. The ecumenical patriarch is considered the "first among equals" of all Orthodox patriarchs worldwide.

The American campaign, organized by the Greek Orthodox lay organization, the Order of St. Andrew the Apostle, began last month with distribution of 1 million pamphlets to churches and other religious organizations across the country that charged "harassment and persecution" of the patriarchate by the Turkish government.

The Greek Orthodox Archdiocese of North and South America supports the campaign. On Sunday, special prayers for the patriarchate were said from the pulpits of the more than 500 Greek Orthodox churches in the United States.

Archbishop Iakovos, primate of the American archdiocese, is asking other church denominations and ecumenical organizations for support. He also met with Secretary of State George Shultz and Secretary of Defense Caspar Weinberger last month on the issue.

Oppression of the Orthodox church in Istanbul is not new, according to the American archdiocese.

The church cited more than four centuries of tyranny under the Ottoman Empire, beginning in the 15th century, including the massacre of hundreds of thousands of Greek Orthodox in 1922 after the switch to the modern Turkish government.

But the current situation, beginning about a decade ago, is crucial, American church officials said this week. "It's the attempt to wipe out the presence of the patriarchate," said the Rev. Alexander Karloutsos, communications director of the American archdiocese.

The Greek minority in Istanbul, where most of the Greek population in Turkey lives, is down from 100,000 in the mid-1920s to 5,000 now, and most of the residents are elderly.

The American church leaders are charging Turkish government violations of human rights and religious freedoms guaranteed in the international Treaty of Lausanne in 1923 and the Helsinki Accords of 1975. The Lausanne treaty guaranteed non-Moslem minorities the same treatment as Moslems and the international status of the patriarchate, American church officials said.

The church's theological college in Istanbul, the major seminary for Greek Orthodox around the world, was shut down by the government several years ago, the church officials charge.

The church is denied permits to make repairs on church property; its printing facilities have been shut down, and the Turkish government has claimed a right to approve candidates up for election as the ecumenical patriarch, Karloutsos said.

In 1972, before the bishops elected a new patriarch, Demetrios I, the Turkish government reportedly rejected several candidates, among them Iakovos, who has been barred from Turkey since the late 1960s.

The church officials charge that Orthodox Christians in Turkey are not permitted to bequeath property to the church or other Orthodox, are denied banking and credit privileges, are taxed at a higher level than others, and if they want to emigrate, are not permitted to take their possessions with them.

Numan Hazar, political counselor with the Turkish Embassy in Washington, said that the charges are "false." He charged the Greek Americans with "political" motives because of conflicts between Greece and Turkey. "We don't have any interest in eliminating the patriarchate," he said.

He said on the closing of the seminary that "all private universities" were closed under a new Turkish law in the mid-1970s.

The Turkish government has taken inventories of the property of "all foundations--religious, scientific, and charity," and owns "all antiquities," including buildings considered to be antiquities, he said. But "nothing was seized," he said.

He said that the government has a right to review candidates for election by the church bishops as the ecumenical patriarch.

Greek American leaders are seeking U.S. State Department pressure on the Turkish government and pressure by groups in other countries, including Moslems.

The U.S. Congress cut off military aid to Turkey in 1975, a year after Turkey invaded Cyprus, but restored the aid in 1978. Neither the Carter nor Reagan administrations has spoken out on the ecumenical patriarchate issue because Turkey is a site of U.S. military bases, the U.S. church officials said.

"They have not spoken out because of our bases there. It's a very difficult situation," said the Rev. Milton Efthimiou, clergy liaison to the Order of St. Andrew. But "all we want is that the seat of our headquarters be accorded those rights that every other church and headquarters is accorded in the world," he said.

U.S. aid to Turkey will amount to more than $500 million this year and more than $700 million next year, making it the third largest recipient of U.S. funds after Israel and Egypt, Karloutsos said.

The patriarchate was officially established in 330 A.D. when the Emperor Constantine moved the seat of the Roman Empire from Rome to Byzantium and renamed the city Constantinople. The city was later named Istanbul.