The veteran leader of Spain's Communist Party, Santiago Carillo, was reelected secretary-general today at the end of a four-day congress that showed unprecedented fissures and internal dissent in the once strongly disciplined party ranks.

A third of the delegates at the congress persistently voted against or abstained on motions dealing with the party's internal organization that were presented by Carrillo's supporters. And in an even more significant break with past tradition, Carillo polled only 687 votes out of 1,056 in the election to fill posts on the Central Committee. The 14-member committee elected him secretary-general with only one abstention.

Criticism against Carillo, 66, who has been the uncontested leader of the party since 1959, came largely from the younger generation of Spanish Communists, who claim the old guard leadership is incapable of implementing internal democracy. The dissidents were joined in key votes by a small percentage of pro-Moscow hard-liners who oppose Carillo's independent, "Eurocommunism" strategy.

[In Moscow today, the Soviet Communist Party newspaper Pravada attacked Carillo's remarks at the congress on foreign policy and the ideological basis of his policies, Reuter reported. Pravada voiced particular anger over Carilla's arguments that superpower rivalries between the United States and the Soviet Union were the chief cause of current world tension, Reuter said.]

At the center of the often angry debates on policies and personaliteis that marked the four-day, 10th party congress was discontent among the leadership and the rank and file over falling membership and the small impact that the Communist Party has been able to make on national politics.

Although it is the third-ranking political force in the country, the party appears to have reached a 10 percent ceiling in national polls and has made little headway in interesting the powerful Socialist opposition in joint electoral pacts.

The main bastion of communist influence in Spain lies in the labor movement, but in union elections earlier this year the Communist-led Workers' Commission only narrowly outpolled their Socialist-organized rival, the Genral Workers' Union.

The main challenge to the leadership came from younger Communists, often university graduates of middle-class background, who formed the backbone of the student revolt against Francoism in the 1960s. In what appeared to be a head-on clash of genrations, the dissidents accused veteran party leaders -- who spent the Franco years exiled in Eastern Europe -- of being inefficient and out of touch.

Although several dissidents were elected to the Central Committee, the opposition movement was broadly dissatisfied with the leadership vote, claiming that the committee did not reflect its strength.

Earlier, in a blistering attack on the creation of factions within the party, Carrillo said internal dissidence would mean the self-destruction of Spain's communist movement.

He said the left in Spain was bent on destroying itself when the extreme right was reorganizing, and that characteristic "contributes to paving the way for a new military coup." Several key dissidents could leave the party as a result of the congress, thereby continuing a series of defections that have rocked the party in past months.

A significant indication of what will succeed the Carillo leadership was the prominence of Nicolas Sartorius, a member of a wealthy and aristocratic Madrid family who was a prominent labor organizer clandestinely and in prision during the latter years of Francoism. Sartorius, 44, who has close links with the dissidents, received an 838-vote endoresement for a Central Committee post, well ahead of the votes polled by Carrillo. He is expected to be appointed deputy leader.

Politically, the Communist Party approved a motion condemning the Spanish government's decision to apply for NATO membership.