Witnesses today reported scores of bodies scattered in the dirt and bushes of what was once a guerrilla camp near the city of Santa Ana, the corpses of young men and women who two weeks ago were intent on launching the "final offensive" to seize power in El Salvador.

The fighting, which began nine days ago as a concerted push to overthrow the U.S.-backed government rapidly dwindled then sputtered to a stop in the face of the Salvadoran population's widespread unwillingness to join the fight and U.S. military aid rapidly being channelled to government forces.

The Salvadoran Army and security forces now are moving out of the outposts they were hard-pressed to defend a week ago and into the countryside on search-and-destroy missions, such as the one that surprised the guerrillas outside Santa Ana yesterday. The government troops are intent on demonstrating that the left can no longer claim control of havens throughout the country.

The fighting has left a grizzly aftermath as the Army and local officials have moved rapidly in several cases to burn the bodies of guerrillas and others who fell in the fighting. Soon after the engagement in Santa Ana most of the corpses were thrown into ditches and burned. Elsewhere in places such as San Francisco Gotera where fighting was heavy, bodies half-burned and half-decayed lay on the city streets for days. Although the incineration of the corpses is intended to stop the stench, in some cases as in Gotera they had to be burned more than once.

The fighting and terrorism -- by rightwing extremists, the guerrillas and the government -- that cost an estimated 10,000 lives last year and at least 1,000 during the last two weeks are not over. U.S. officials, however, are optimistic that the violence will decline when the rightist extremists recognize the goverment's determination to end the violence and the guerrillas, or at least some of their followers, sense defeat.

But the insurgents are still capable of sporadic actions or could opt for some spectacular, violent stunt against a single target. They could also find the resources to launch another push in the near future.

Only this particular "final offensive" can be said to have ended. Salvador's President Jose Napoleon Duarte considered San Francisco Gotera, the capital of Morazan Department and the object of some of the guerrillas' strongest attacks last week, so safe now that he walked its streets among crowds of residents yesterday.

In the wake of the offensive, however, serious changes have taken place in El Salvador that may have farreaching effects for the entire area.

The most conspicuous is the delicate confrontation between the United States and Nicaragua, which was brought into the open last week with State Department charges that the Sandinista government is heavily aiding the Salvadoran guerrillas. The object of the charges was the supply of sophisticated arms the guerrillas are said to have received by way of Nicaragua in the last few weeks and months.

But at the time that Washington announced renewed of "non-lethal" military aid to the government last week, U.S. Ambassador Robert White also charged that about 100 guerrillas had landed in El Salvador on boats from Nicaragua.

That some soldiers landed and engaged government troops is certain, but where they came from and who they were has not been established, and U.S. officials here now are saying White "overemphasized" the supposed invasion and no longer thinks the evidence about it as "compelling" as the day he spoke.

Nicaragua, which by U.S. law would face a complete cutoff of aid if it is proved to be helping guerrillas in other countries has steadfastly denied all charges of its involvement in El Salvador.

In the meantime, however, Washington is sending six helicopters and, since Saturday, "lethal" equipment including M16 automatic rifles, grenade launchers and ammunition to the Salvadoran government.

The actual supply of guns and ammunition is unprecedented in the recent history of U.S.-Salvadoran relations and comes without any results being shown thus far from the investigation into the killing of four American churchwomen last month that prompted the original cutoff of all military supplies.

Human rights and church groups charged that the mere fact of an investigation hardly justifies the sending of arms, which conservative Salvadoran military officials may see as an endorsement of their past, often extraordinarily brutal practices.

The pressure is also being taken off the military to negotiate a peaceful solution to the chronic political carnage, critics say. But the government at least has said repeatedly that it is willing to negotiate while the guerrillas of the Farabundo Marti National Liberation Front and their political allies in the Revolutionary Democratic Front continue saying they will not deal with this government, but only with the United States.

Both U.S. and Salvadoran officials say they regard this as a tactic to undermine the government, not a serious proposal.

The church, which appeared at the vanguard of revolutionary fervor before Archbishop Oscar A. Romero was assassinated last spring has since been steadily looking for a middle road between the government and guerrillas under Romero's acting successor, Bishop Arturo Rivera y Damas. In his Sunday homily Rivera y Damas cited four criteria for legitimately supporting revolution -- that the government abuses power, that peaceful means have been exhausted, that the revolution will replace the current government with something better and that the revolution stands a real chance of succeeding.

Only the first criterion has been met here, said Rivera y Damas. "In this country there have been great abuses of power."

But there are still peaceful means of resolving El Salvador's problems and the Salvadoran people are not convinced that the revolutionaries offer either a better future through insurrection or a real chance of winning, Rivera y Damas told his congregation.

In the propaganda war outside the country the left is claiming this was never intended to be a final offensive and there were no deadlines set despite the flat announcements by guerrilla leaders in recent weeks that U.S. President-elect Ronald Reagan would be presented with an "irreversible" military situation in El Salvador by Inauguration Day.

In a sense they appear to have been right. The Salvadoran Army, with its new American equipment and its confidence bolstered by the last two weeks of fighting, is apparently stronger than ever.