Veteran center-liberal politician George Mavros has announced that he will run on a Socialist ticket in the Oct. 18 national elections.

Mavros' decision, formulated in an exchange of letters with Socialist leader Andreas Papandreou, is regarded as a major boost for the Panhellenic Socialist Movement (Pasok), which is strongly challenging the rightist New Democracy party of Prime Minister George Rallis.

Mavros, 72, whose social democratic Union of Democratic Center ran third in the 1977 elections in which Pasok emerged as the major opposition party, said in his letter that he "gladly" accepted an invitation by Papandreou to join Pasok "in the great struggle to end right-wing rule and bring about the change demanded by the people."

In a later statement, Mavros called on all centrists to "join the fight for victory and change."

"Change" is the campaign slogan of the Socialists. The decision by Mavros to run on a Pasok ticket drew sharp criticism from New Democracy, which accused the centrist politician of "adopting a flag of convenience" in the October elections.

Papandreou welcomed Mavros' move, saying a united effort by all the democratic forces was necessary to oust the right. Mavros, who during the past months has tried unsuccessfully to unite the fragmented center forces under the umbrella of his new Center Rally Party, is expected to draw with him many of the voters who supported him in 1977. Four years ago his party captured 12 percent of the vote.

Center votes could tip the scales in next month's election, which is expected to be close. A nationwide poll published this week showed Pasok ahead by six percentage points (40 percent against 34 percent for New Democracy).

Mavros, who succeeded Papandreou's father, George, as leader of the Center Union Party on the elder Papandreou's death in 1968, was a staunch opponent of the military junta that ruled Greece between 1967 and 1974.

Mavros' moderate positions on such issues as Greece's membership in the the European Community is expected to help dispel Pasok's extremist image. Pasok is trying to present the Socialists as bearers of risk-free change.

Mavros was opposed to Greece's rejoining NATO's military wing because of the continued Turkish occupation of 40 percent of Cyprus. But he has in the past favored a free-enterprise economy, and he is strongly committed to Athens' membership in the European Community.

Papandreou seeks renegotiated terms under which Greece joined the European Community and major concessions if Greece is to continue membership in NATO's military wing.

He has, however, ruled out cooperation with the Communist Party, which is against membership in the European Community and NATO.