With special military units mobilized to defend polling places against threatened guerrilla attacks and the four presidential candidates making their final campaign pitches under the protection of bodyguards, Guatemalan President Romeo Lucas Garcia sought to assure foreign observers today that Sunday's election will be free of fraud.

Clearly stung by expressed skepticism about such prospects, and under mounting U.S. pressure, Garcia called ambassadors and journalists to election headquarters to explain the mechanics of the voting.

"I have brought you here today so you will be well-informed and will not be influenced by all the talk that these elections will be fraudulent," the president said in the auditorium where the returns are to be tabulated. "I want you to know that they will be pure, free, and clean."

Gen. Garcia's surprise gathering to defend elections, as well as his four-year rule, came a day after U.S. Ambassador Frederic Chapin expressed Washington's hope that a democratic election would give the country a government able to curb violence that has resulted in the death of thousands of civilians.

In a speech to Guatemalan businessmen, Chapin called this "a transcendent moment in the history of Central America." He indicated that any U.S. help for Guatemala's economy and its hard-pressed armed forces was contingent on changing the repressive policies.

Honest elections and turnover of power to the victor, Chapin said, would be one of the "self-help" measures that Washington would be looking for as a sign of change that would allow the Reagan administration to consider extending help to Guatemala under the new Caribbean Basin initiative.

Chapin added, "Another self-help measure is the elimination of violence against third persons or noncombatants in the necessary war to eliminate the threat from communist-supported insurgents."

Chapin alluded to civilian deaths attributed to right-wing "death squads," supported or tolerated by the government, that have escalated since Lucas was elected as the candidate of the armed forces in 1978.

Church sources here put last year's death toll at 11,000 and the U.S. Embassy estimated an average of more than 300 a month. Guatemala's human rights record has isolated it from Washington as well as most of Latin America.

Gen. Lucas is backed by the coalition of conservative civilians and military that dates to a CIA-sponsored coup against leftist Jacobo Arbenz in 1954. He has depicted the violence as the result of the guerrilla campaign that he insists is orchestrated by Cuba.

Lucas has rejected diplomatic efforts to encourage curbing the violence against civilians, despite the increasing need for aid.

That attitude has reinforced skepticism here about the prospect of honest elections Sunday. There are four candidates for the presidency: Gen. Angel Anibal Guevara, of the ruling coalition; the far rightist former vice president, Mario Sandoval Alarcon; a moderate supported by the Christian Democratic Party, former minister of education Alejandro Maldonado; and architect Gustavo Anzueto, the candidate of a former military president whose influence has since waned.

With the center-left forces decimated by assassinations and exile, and the centrist Christian Democrats intimidated by the death of 238 of their party officials in the past 18 months, the common wisdom until recently was that the elections would be a contest among rightists who would continue the hard-line policies of Lucas.

As the election has neared, however, hopes that it would be honest and significant blossomed among the candidates as well as members of the electorate.

These hopes have centered on the candidacy of Maldonado, who is depicted by many seemingly disinterested observers as representing a real change. The fact that the government has allowed his candidacy, and that he has campaigned around the country with minimal intimidation and threats, has further encouraged even skeptics.

The government also has sought to encourage international scrutiny of the election, opening its doors to the international press, which Gen. Lucas said today would serve as observers of the fairness of the election.

Politicians and diplomats long familiar with the government's methods have taken a wait-and-see attitude. But privately they voice hope that Gen. Lucas' international isolation has convinced him, and more importantly, his supporters, that change is imperative if Guatemala is not to be plunged into civil war.

The Guatemalan establishment has been shaken by the fact that Ronald Reagan's election--greeted with firecrackers and street dancing here--has not helped bail out their country because of continuing U.S. congressional criticism.

Guatemalan military men are in desperate need of helicopters and planes to fight the insurgency in the countryside, as well as parts for those bought before U.S. military sales were cut off in 1977.

Equally important has been an economic crisis as tourism and credit have dried up because of the instability. Guatemala was forced to use its last foreign exchange reserves last month when a bank consortium refused to roll over a $75 million loan that came due.

The Reagan administration promised two weeks ago $250,000 in economic aid, and possibly more later, if the rights record improves. Many here say this may have convinced Gen. Lucas and his backers that real elections must be allowed.

Even such an election would not necessarily rescue the country from crisis. The election of Gen. Guevara, the former defense minister, or Sandoval, who long has supported the country's hard line, would be seen by their opponents as a continuation of the present regime and could jeopardize what is seen as a necessary second stage.

Chapin, for instance, insisted that the election are but one step toward international acceptance. Curbing violence remains the second difficult step. Supporters of Maldonado say his election would encourage that change.