Thousands of Salvadorans, rich and poor, marched through this riot-torn capital today to pay a tearful homage to assassinated archbishop Oscar Arnulfo Romero as armored cars and heavily armed police enforced an uneasy peace.

There was little doubt that the murder yesterday evening was carefully planned and apparently carried out by professional killers. It left this small Central American nation in a state of shock mixed with fears that more violence would soon come.

After a night of numerous bomb explosions, apparently reflecting anger at the killing of the 63-year-old Roman Catholic leader, many shops in the capital were closed today and many people stayed home, bringing an unaccustomed calm to the otherwise bustling downtown area.

Romero was shot while saying mass at a small chapel in northwestern San Salvador. A fragmenting .22 calibre slug struck him directly in the heart just as he had concluded his homily with remarks about death and the need to give one's life to the cause of peace.

Although theories abound, the exact motives of the killer, who escaped in a car with several other men, remained unclear. Some church officials close to the archbishop said they believe that the murder was the work of right-wing extremists, who have frequently been the objects of Romero's impassioned oratory and who may be anxious to provoke a popular insurrection that could then be crushed decisively.

"The [marxist organizations of peasant and workers] have conceded that the moment has not yet arrived for revolution and I hope that this will not pull them out of their position," said one politically active priest.

Several observers noted that Romero's last major public address on Sunday had specifically called for soldiers to refuse to obey orders that ran contrary to God's will. "I ask you, I pray you, I order you in the name of God," Romero concluded, "stop the repression."

A few hours before Romero was killed, a spokesman for the armed forces termed Romero's exhortation to the troops "a crime."

Some sources speculated, in the convoluted reasoning of Salvador's violent politics, that the same leftists with whom Romero had shown increasing sympathy killed him so as to place the blame on the rightists and win greater support for their revolutionary cause.

But for the thousands of mourners who crowded into the city's Basilica of the Sacred Heart, crying before the archbishop's coffin and cheering uncontrollably as his words of support for the poor and oppressed were read to them, his death was the most devastating example of the tragedies that have beset this tiny country.

Children and parents, old women and intense young men pressed together, singing hymns beneath the Basilica's lofty rafters.

"He was our father," said one middle-aged man. "He was the only real defender of the people. We were hoping that things would be getting better. But now, who knows? This is the worst."

A priest described the damage wrought on El Salvador's already divided church by Romero's death as "irreparable, absolutely irreparable. We say to the faithful that the church doesn't depend on one single person, that it will go on. But we have been hit very hard."

The morning's papers were filled with denunciations of the murder presented as full-page advertisements by the ruling military-civilian junta and the armed forces.

U.S. Ambassador Robert E. White called the assassination "a crime against the people of El Salvador and the world" intended to "generate more violence in order to bring down the government and create chaos."

Though there were a dozen bombings late last night after news of the murder spread throughout the city, there were apparently no injuries. Many people, on hearing of the assassination, had fled immediately to the security of their homes.

Although the situation is calm at the moment, few people doubt that the murder of the archbishop is likely to result in further bloodshed, adding to the toll of more than 700 lives lost to political violence since the beginning of the year.

There are plans for several marches during the course of the week, including the bearing of the archbishop's coffin from the basilica to the National Cathedral Wednesday morning. The funeral is not expected to take place until Sunday at the earliest. The potential for confrontation remains high, despite the fact that the archbishop spent most of his life advocating peace.

"If the murderers have lost respect for the signor [Romero]," said one observer at yesterday's mass, "then now nobody is safe."