A nationwide Greek Orthodox Church council has denounced government plans to legalize civil marriages, bringing into the open differences between the conservative church and the new Socialist administration of Prime Minister Andreas Papandreou.

The statement by the church council may well signal the start of a power struggle between the Greek church and state, testing the influence of the centuries-old and traditionally powerful Greek Orthodox clergy against that of the Socialist Party (Pasok), a new political force that swept last October's general elections on a platform of civil law reforms and the separation of the church from the administrative workings of the state.

"According to the teachings of our church, civil marriage is tantamount to prostitution and adultery" said a statement issued after the reportedly stormy council meeting Wednesday. "Any Greek who marries in this fashion automatically sets himself outside the church and forfeits its blessing." It was signed by Archbishop of Greece Seraphim and 75 metropolitans from all parts of the country.

The council said that it will recognize civil marriage as an option only for those Greeks prepared to declare themselves atheists. It cautioned the government that demoting religious marriage from its current compulsory status would endanger "the unity of the nation."

The council's sharp criticism of the government reform proposals, which had been discussed and, it was widely thought, agreed upon in mixed church-state committees, is viewed here as a victory for hard-liners inside the church. Their fear is that the influence of the church will be weakened by the proposed revision of the civil and family law codes.

The revision, aside from introducing civil marriage, includes striking adultery (now punishable by up to a year in jail) from the list of criminal offenses and changing the article of the codes that states that the husband is the head of the family.

The church avoided taking a stand in the October election battle, in keeping with its tradition of bending to the prevailing political winds. But it is still a powerful symbol for many Greeks, who are taught in school that it was the church that, in underground schools, carried the torch of Greek language and culture through 400 years of Ottoman rule.

Speaking to the press after the council meeting, Greek Justice Minister Efstathios Alexandris said the final arbiter on the issue will now have to be Papandreou. The prime minister, who is not noted for a religious temperament, so far has tried to accommodate the sensibilities of the religious authorities by stressing that the separation of church and state that he is ultimately committed to will not disturb "the ties of the church with the nation and the people."

Part of his caution can be attributed to the fact that the church remains particularly influential in rural areas, where a voter swing away from the former ruling party New Democracy played a crucial role in the Socialists' election victory.

The prime minister, however, also will be under pressure from Greek women's organizations, which rallied female voters in support of the Socialists last fall on the basis of pledges to usher in long-requested changes in civil and family law. Under the former administrations of New Democracy, the government was anxious to avoid antagonizing the church by acceding to feminist demands.

Former prime minister George Rallis, whose campaign poster featured a chapel in the background, had assured the Holy Synod that his administration would not legalize civil marriage.

If the Socialist reforms go through, they will end the irregularity caused by the fact that Greeks married in a civil ceremony abroad are not considered legally married in Greece. They thus revert to single status and are free to marry again once inside the Greek border.