Greek Premier Constantine Karamanlis yesterday called parliamentary elections for Nov. 20, a year ahead of schedule, in an apparent effort to win a new mandate that will allow him to deal forthrightly with pressing foreign-policy issues.

The elections, announced in Athens after a meeting between Karamanlis and opposition leader George Mavros, expected to give the first clear poltical reading of the Greek electorate since before the eight-year military dictatorship that collapsed in 1974.

With an expected new mandate in Parliament, Karamanlis would have a stronger hand in dealing with the major problems confronting the strategic Mediterranean country: achieving full membership in the European Common Market and resolving the potentially explosive disputes with Turkey that threaten the eastern security of NATO.

Diplomatic sources said that by calling elections now, Karamanlis apparently intends to take advantage of Greece's relatively quiet internal situation as well as of a growing disarray among the opposition.

All indications are that Karamanlis' popularity has slipped only slightly, if any, since his New Democracy Party won 54 per cent of the vote in the November 1974 elections, which were held in the highly charged atmosphere immediately after the downfall of the military junta.

Major opposition leaders, who opposed early elections, had won Karamanlis' agreement to give sufficient notice to allow for a meaningful campaign. The present Parliament is expected to be dissolved by presidential decree about Oct. 15.

A key question to be answered the coming elections is the relative strengths of the factions that now form the opposition.

The Democratic Center Union, headed by Mavros, won 20 per cent of the vote in 1974 but is expected to split into two wings this time, making it unlikely that Mavros could continue to head the parliamentary opposition.

The Pan-Hellenic Socialist Movement(Pasok), headed by Andreas Papandreou, a harsh critic of the United States, was the strongest leftist party in the 1974 elections, with 13 per cent of the vote. Analysts expect it to do at least as well in new elections.

Despite talks among leaders of some opposition factions, efforts to form a united opposition front to run against Karamanlis have been unsuccesful.

The opposition's chief criticisms of Karamanlis center on what they call his government's inadequate handling of problems in the economy, labor, education and social services. Only Panpandreou's party and the Moscod-oriented Communist Party oppose Greece's entry into the Common Market, a chief goal of Karamanlis.

There is some speculation in Athens and Washington that Karamanlis, by calling early elections, might be setting the stage for assuming the presidency.

The post-junta constitution adopted by Greece in 1975 created a strong presidency. The president, elected by a two-thirds vote of Parliament, has power to name and dismiss Cabinet members, including the premier, and veto legislation.

The current president, Constantine Tsatsos, was elected to a five-year term in June 1975. Karamanlis has indicated that he is interested in assuming the presidency when Tsatsos' term ends, but to do so he would need a firmer control of Parliame nt.