Western European pressure on Turkey's military government to return the country to democratic rule was increased last night when the 21-nation Council of Europe ordered an official investigation of alleged human rights abuses in Turkey.

In a strongly worded resolution, delegates from the other 20 countries of the council, which promotes democratic values and human rights in Europe, accused Turkey's military rulers of human rights violations ranging from the abolition of political parties to torture of prisoners.

The resolution, approved by a 68-41 vote during a council assembly in Strasbourg, France, also urged the Turkish authorities "to free political prisoners, allow democratic and trade union organizations to reconstitute free from interference, and to reestablish democratic institutions."

The delegates defeated a proposal by Italian Communists and Greek Socialists to expel Turkey--which had joined the council two months after its founding in 1949--despite a critical report from 20 delegates who visited Turkey earlier this month. The council's official investigation is expected to take at least several months and could result in another move to expel Turkey.

The military dictatorship that ruled neighboring Greece, another NATO member, from 1967 until 1974 withdrew from the council under threat of expulsion in 1969 after an investigation by the council's European Human Rights Commission. European diplomats cite the Greek case as a precedent for their hostility toward the Turkish military government of Gen. Kenan Evren, despite its membership in the NATO alliance and its efforts to join the European Community.

The Common Market suspended its program of economic aid to Turkey last November. Its European Parliament called last week for the aid suspension to continue "until respect for human and civil rights and democratic liberties is once again restored" in Turkey.

The Reagan administration, which has increased military aid to Turkey as part of efforts to strengthen NATO defenses, has pressed the Europeans to relax their stand on Turkey and resume economic aid. Thus Turkey joins the growing list of disagreements between the United States and its European allies.

In an exchange widely commented on in Europe, Secretary of State Alexander M. Haig Jr. was asked by a British reporter at a press conference for European journalists in Brussels earlier this month whether the United States was using a "double standard" in condemning martial law in Poland and urging sanctions against it while increasing aid to Turkey.

Haig responded with visible anger that the question itself "reflects a double standard" by Europeans equating Poland and Turkey "that boggles my mind."

Haig asked his questioner whether he had forgotten that before the military took over "up to 30 people were being assassinated each day by terrosists" in Turkey. Commentators in Europe, including relatively conservative ones, pointed in answer to estimates that 30,000 people have since been detained by Turkey's military rulers. They also suggested that the hostility of the new Greek Socialist government of Andreas Papandreou to the United States and NATO could be attributed to U.S. support for the earlier military government in Greece.