San Salvador's Catholic archbishop, one of the most powerful political voices in this violently divided nation, warned President Carter today that U.S. or other outside interference in the affairs of El Salvador would lead to "a major bloodbath in this longsuffering country."

Archbishop Oscar A. Romero, 62, specifically referred to reports that the United States plans to send up to $7 million in military equipment as well as Army advisers to help prop up the Central American country's armed forces and its faltering military-civilian junta.

"The contribution of your government," Romero said in a letter mailed to Carter this morning, "instead of favoring greater justice and peace in El Salvador, undoubtedly will sharpen the repression."

[In Washington, administration sources said the military aid program was approved in principle by the National Security Council Friday. They said the Salvadoran junta, whose civilian members initially hesitated in accepting the plan, agreed on condition that it be a multilateral effort in conjunction with other Western countries. There was no indication, however, of when the program would be implemented.]

U.S. shipment of $200,000 worth of tear gas, gas masks and bulletproof vests, as well as the temporary placement here last November of a six-man U.S. military training team to instruct Salvadoran police in riot control already have contributed, at least indirectly, to a major increase in bloodshed, according to Romero.

"The security forces, with better personal protection and effectiveness, have repressed the people even more violently, using deadly weapons," he said.

According to the Catholic Church, more than 50 people have been killed in the first two weeks of February alone. A recent report issued by the local human rights commission listed 194 deaths from political violence between Jan. 22 and Feb. 8.

Leftists demanding an end to repression, improved living conditions for the poor and the release of jailed comrades have occupied a series of government buildings and diplomatic missions here in recent weeks.

Today 50 members of the leftist-dominated National Teachers Association seized part of the Education Ministry and held almost 200 persons.

Later, other leftists ended their occupation of the Spanish Embassy and freed their two remaining hostages unharmed.

Romero, who has advocated peaceful conciliation between left and right political extremes here in the past, has taken an increasingly revolutionary posture in recent weeks as the current coalition government of Christian Democrats and young military men has appeared nuable or unwilling to control rightist factions in the armed forces engaged in an all-out effort to suppress the left.

"Christians do not fear combat, they know how to fight," Romero told a Mexican journalist last week, "but they prefer the language of peace.

"However, when a dictatorship gravely attempts to go against human rights and the common good of the nation; when it becomes insupportable and closes the channels of dialogue, of understanding of rationality; when this occurs, then the church talks of the legitimate right to revolutionary violence."

Nominated last year by British members of Parliament and some U.S. congressmen for the Nobel Peace Prize, Romero has been this country's most outspoken defender of human rights and advocate of redistribution of income.

A spokesman for the archdiocese said today that Romero still hopes a way may be found to avoid a violent revolution. The spokesman held out the possibility, however, that the church would not actively oppose an insurrection that brought demonstrably popular forces to power.

Romero's public statements have reflected increasing disillusionment with the current government since the beginning of the year.

But the reforms have been slow in coming even since the reformist Christian Democrats joined the junta.

Romero, in his letter, addressed Carter as "a Christian" who has "shown that you want to defend human rights." The letter, which he read to an overflow crowd at mass yesterday, also indicates the extent to which his own pessimism about the situation in El Salvador has deepened.

Referring to the takeover of the Christian Democratic headquarters by the left-wing Popular League Feb. 28, and their bloody ouster last week, Romero said:

"The brutal form in which the security forces recently dislodged and murdered the occupants . . . despite the fact that the junta and the party did not authorize the operation, is evidence that the junta and the Christian Democrats do not govern the country, but that political power is in the hands of unscrupulous military men who only know how to repress the people and favor the interests of the Salvadoran oligarchy."