The Vatican today denounced as "incredible" the firing-squad execution this morning in Guatemala of six prisoners, only three days before a scheduled visit there by the pope and despite an appeal for clemency from the Holy See.

Pope John Paul II, who in an address here today urged sanctions against human rights abuses, expressed in a statement issued by the Vatican "all his most profound sadness" on hearing the "dramatic, unexpected and incredible" news that President Efrain Rios Montt's government had shot the six men at dawn.

The executions came as the papal nuncio in Guatemala, Oriano Quilici, was seeking an urgent audience with the president in the pope's name to prevent the sentences from being carried out, the statement released here said.

The Vatican warned that the executions could have "grave repercussions" in Guatemala and the world "and also at the level of even the Holy See." Although this seemed to stop short of a cancellation of the pontiff's visit Monday to Guatemala, it cast a shadow of recrimination over a trip that the pope had been seeking to portray as a message of love, fraternity and peace-making.

The six prisoners, five Guatemalans and a Honduran, had been found guilty of kidnaping and "terrorist crimes" and sentenced to death on Feb. 2 by special tribunals organized by Rios Montt to facilitate his fight against leftist guerrillas.

Gen. Rios Montt describes himself as a born-again Christian. Since he took power after a coup almost a year ago, fundamentalist sects that already were growing have flourished in Guatemala--often at the expense of the Catholic Church that traditionally has dominated religion in Central America.

A number of international human rights organizations, including the Organization of American States human rights commission, protested the executions. The Vatican previously had expressed the pontiff's dismay over at least four earlier executions ordered by the special tribunals. In honor of the pope's awaited visit to Guatemala, guerrillas there had said they would observe a three-day cease-fire.

Reuters reported from Guatemala City that the executed were Carlos Subyuc, Pedro Tepet, Marco A. Gonzalez, Hector H. Morales Lopez, Sergio R. Marroquin and his brother Walter V. Marroquin.

News of the executions arrived here shortly before John Paul declared in an address on human rights that Central America must have effective means for verifying rights abuses and "appropriate sanctions" for those who violate them.

The pontiff made his remarks in an address to the OAS Inter-American Court of Human Rights at the close of his first full day in a tour that is also to include Panama, Nicaragua, El Salvador, Honduras, Belize and Haiti. As he has from the moment he touched down here yesterday afternoon, John Paul approached the politically charged issue in words described by regular Vatican observers as unusually explicit, even for this socially committed pope.

"This institution, which not without good reason has chosen San Jose as its headquarters, demonstrates a lively realization by American people and rulers that promotion and defense of human rights are not a mere ideal . . . but rather that they should have at their disposal effective instruments of verification and, if need be, appropriate sanctions," he said. The democratic Costa Rican government is often cited as having an exemplary record on human rights in the troubled region.

On one level, the pope was offering an endorsement of the fledgling human rights court, which has yet to decide a case. His message mentioned none of the Central American countries--such as El Salvador, Nicaragua, and Guatemala--where human rights have been a contentious national and international issue.

On another level, however, his plea for effective verification procedures and sanctions against violators paralleled calls in the U.S. Congress. It has sought to condition U.S. aid on more stringent observance of human rights.

At the same time, the pope appeared to echo appeals from the Reagan administration and Nicaraguan exiles for increased protection of such human rights as free speech by the revolutionary Sandinista government in Managua.

John Paul, celebrating an outdoor mass in San Jose's spacious Sabana Park, again urged Central America's Roman Catholics to make their church a guiding force not only for their spiritual lives, but also for their everyday political and social lives. As he spoke before a crowd of several hundred thousand, applause rose several times from the throng.

"The church, with its doctrine and example, that of its saints and teachers, exhorts us to take care not only of spiritual things, but also of the realities of this world and the human society of which we are part," the pontiff said.

"It exhorts us to commit ourselves to eliminate injustice, work for peace and triumph over hatred and violence, to promote the dignity of man, to feel responsible for the poor, the sick, the alienated and oppressed, refugees, exiles and displaced persons, as well as so many others in need of our solidarity."

As the pope moved to and from visits to the National Children's Hospital, the residence of President Luis Alberto Monge Alvarez, and the cathedral, Costa Ricans turned out by the thousands to cheer his passage. His presence in elegant white robes and his stately gestures of blessing and greeting generated a festive atmosphere in the streets, with people straining to see as they would for a rock star.

But for those who listened closely to his words, the ponfiff also seemed in his addresses to be developing a carefully structured appeal for Central America to turn to the teachings of traditional Roman Catholic faith as a basis for increasing social justice and reducing the level of violence that has turned much of the isthmus into a zone of conflict.

Repeatedly he proposed the church's message as the answer to tumult here rather than the authoritarianism of the right or the totalitarism of the left.

"Go ahead; do not become discouraged by the difficulties," he told the congregation at mass. "Do not forget the Christian values that distinguish you and that have supported you until now. Be faithful to our tradition and aspire to be a model of just social organization in these moments of profound changes and serious challenges."

In a later talk to Central American nuns, however, he emphasized church policy barring clergy from direct participation in politics and warned against turning to ideologies "that one day could demand from you the price of your own liberty."

This was interpreted as an indirect reference to the situation in Nicaragua. John Paul has urged the resignation of priests serving in the Marxist-dominated Sandinista government, including Foreign Minister Miguel d'Escoto and Culture Minister Ernesto Cardenal. So far, however, his appeals have gone unheeded.