After the narrow victory by Greece's Prime Minister Constantine Karamanlis in last Sunday's election. Europe's hitherto strongest parliamentary majority is a lot less solid today.

There is no question in the minds of diplomatic officials that the powerful political personality of Andreas Papandreou, new leader of the opposition, will have an impact on the conservative policies of the Karamanlis government.

Papandreou, 58, a fiery, Socialist who toned down much of his rhetoric during the election campaign, was the fulcrum for a surprise surge of the Greek left which captured 38 per cent of the ballots, its greatest electoral triumph in the post-war years. There will thus be a political opposition which, for the first time in mordern Greece, is diametrically opposed to the government on all major foreign issues.

One diplomatic official said: "Papandreou has brought a whole new ballgame into play."

He said Papandreou "will provide a real opposition, and karamanlis can no longer ignore Parliament as he has in the past. Papandreou is bound to have a restraining influence on the government's foreign policy . . . And this puts Karamanlis' hopes for negotiations and realistic concessions on Cyprus and the Aegean considerably back."

Papandreou reiterated the message in an interview this week: "It is very clear that whatever plans Karamanlis has concerning his foreign policy will have to be revised . . . On Cyprus and the Aegean, Washington has supported Turkish expansionism, and pressured the Karamanlis government to behave responsibly: In other words not to react. I can assure you that through our voice in Parliament, the voice of Greece will be heard . . . We are a genuine opposition this time."

Papandreous, former head of the economics department at the University of California at Berkeley and son of a former Greek premier, presented a mixed message of militancy and moderation during the election campaign. He is against any concessions to Greece's adversary Turkey and is the country's most outspoken opponent of American foreign policy today.

"NATO has meant for Greece a seven-year dictatorship . . . It is therefore not surprising that in Greece, where American policy has had a greater impact than in other European countries, the majority of the people are not pro-NATO," Papandreou said. "We want Greece out of the alliance and the American bases must go . . . there can no longer be a clientstate relationship between Athens and Washington."

If there is one point on which Papandreou and sources close to the Greek Prime Minister agree, it is that opposition to American policy in the eastern Mediterranean played a definite role in the gains of Papandreou's Panhellenic Socialist Movement (PASOK), which received 25.5 per cent of the vote in the parliamentary race.

Papandreou continued: "Washington can play a constructive and positive role in solving the present crisis between Greece and Turkey over the Aegean Sea. Whether it will, however, is another question."

If Washington were to put its full political and diplomatic weight behind the implementation of the United Nations' resolutions on Cyprus and guarantee the inviolability of Greek frontiers. remove its nuclear weapons and tracking stations from the country and "cease interfering in Greece's internal affairs," then, according to Papandreou, "a Greek government, including a future PASOK government, would be able to normalize its relations with the United States . . . "

Diplomatic officials believe the leftists' gains in Sunday's elections signaled voters' insistence on real economic and social change and spawned a new two-party system in this country along modern European lines. There is now a well-defined distinction between Conservatives and Socialists. and with the Communist Party edging close to 10 per cent, the new political patterns in Greece are not unlike those of France.

"The Communist Party, and to a much greater extent PASOK, have built up highly effective grass-roots organizations since the last elections," political observers said. "They have concentrated on issues and overthrown many of the old political fiefdoms that controlled the vote in the past. Papandreous has cut into the conservative rural areas, which have never deviated from the right."

An ambitious PASOK program for social and economic reform poses an additional challenge to the government of PASOK."

Karamanlis, 70, is also faced with the pressing problem of forming a new government after six incumbent ministers went down to defeat. He must also reorganize an ideologically varied party which can survive his own retirement from the political scene.

Some diplomatic officials feel that, faced with a leftist-dominated opposition, Karamanlis may be forced further to the right. They point out that the monarchists in his party, who did well in Sunday's vote, will now have a greater voice in shaping policy for the government of Karamanlis.

[Reuter reported that George Mavros, whose Union of the Democratic Center Party polled only 12 per cent of the votes in Greece's general election, announced his resignation as party leader today.]

[In the 1974 election Mavros' party won 20.5 per cent and formed the main opposition party with 57 seats in the 300-seat parliament.]