Guatemala's junta off to a fast start, but will it last?

''The longer he stays, the stronger he will become,'' says a Western diplomat.

But ''if he's still making spontaneous and shallow speeches about peace and love six months from now, and not improving the economy, he may not last too long,'' remarks a Guatamalan university student.

The two comments provide clues to Gen. Efrain Rios Montt's ability to hold onto the power he was handed on a plate by the six junior Army officers who staged the March 23 coup and ousted Gen. Romeo Lucas Garcia.

Because he was not elected, General Rios Montt has had to concentrate first on seeking wide public acceptance. He has been, in effect, a ''candidate'' for continued power as well as president of the ruling military junta.

And while wooing the public at large, he has had to cultivate the all-important confidence of the military, not to mention placating the country's politicians.

Hence, if he can carry out some of his early pledges of peace, a healthy economy, justice, and an end to corruption and political killings, analysts here say, his chances of remaining in the National Palace will greatly increase.

So far, he seems to be having some success. But it is not yet clear how much of the gain is public relations and how much is substance.

In the junta's first month in office, political killings in Guatemala City and in the surrounding regions - estimated at 250 to 300 per month during 1981 by the United States Embassy here - have dropped dramatically, according to Guatemalan and foreign officials.

''You should have seen the thugs that used to cruise around in unmarked jeeps with blackened windows and the rear doors open,'' says a resident in the capital. ''Now they've completely disappeared.''

These were the feared judiciales, a group that has been dismantled by the government since the coup. Some of its members have been jailed; others have been recirculated into the nation's security forces.

Some 2,000 policemen who served as bodyguards to Lucas Garcia officials have been returned to regular duty as well. The detective force has been altered, too: now policemen are the only officials allowed to make arrests.

The junta also immediately set up an office in the National Palace to help families trace desaparacidos (missing relatives) - something unheard of during the Lucas Garcia government. ''People were just too scared to go to anyone,'' says a Guatemala lawyer.

General Rios Montt's pledges of eradicating political corruption have brought visible results as well. Some 30 ex-officials of the previous government have been arrested amid much publicity. They are awaiting trial.

As a further gesture, Guatemalans who were forced to leave during the Lucas Garcia regime have been invited back by General Rios Montt. ''I am no avenger, no judge. All Guatemalans who are abroad, forced by personal circumstances under the previous regime, may return to the country and begin to work in favor of Guatemala,'' he said.

Aside from these initial steps, however, the junta has been evasive about setting out specific government policies. A much-discussed list of statutes to replace the nation's suspended constitution has yet to appear.

But the general public does not appear to be clamoring for immediate changes or concrete plans for the future.

Watching Rios Montt address the nation on television Easter Sunday, for example, a young Guatemalan engineer was not amused - as was an American colleague - by the slightly comical combination of the retired general's business suit, palm trees blowing in the wind, and marimba music playing in the background.

''Finally we have an honest leader. Someone to trust in,'' the engineer said earnestly, without looking away from the television screen.

In the weeks since that Easter address, the junta has made a concerted effort to come out of its previous isolation. Almost daily it holds public relations exercises with key groups in Guatemalan society.

The three-man junta of General Rios Montt, Gen. Horacio Maldonado Schadd, and Col. Luis Gordillo Martinez has conferred with university professors, officials of the Roman Catholic and evangelical churches, political parties, and the diplomatic corps.

(Some 34 nations have recognized the junta at this writing, and the US ambassador, Frederic Chapin, has said he is willing to vouch for the junta before the US Congress. The US is also considering resumption of US military aid for the first time in four years.)

The junta's effort seems to have reassured elements of society and diplomats that they had not been forgotten.

The April 15 meeting with leaders of Guatemala's moderate political parties (opposition parties in the last election) was held to discuss the nation's future and to suggest ideas for new electoral laws. The parties supporting Gen. Angel Anibal Guevara - the winner of the March 7 presidential election who was thwarted from taking office by the coup - did not attend the conference.

One diplomat here expressed concern that the junta might be dividing the country by not inviting the United National Front, the Institutional Democratic Party, and the Revolutionary Party because of their connection to General Guevara. ''The political parties represent the people, and this is a time when the junta especially needs national unity,'' the diplomat said.

The junta's Colonel Gordillo insisted, however, that all political parties were invited.

But their absence still leaves a question mark hanging over their support of the junta. But the political parties that met with General Rios Montt, generally approved of the junta.

''We discussed issues in the people's interest and not in the interests of specific groups,'' said Gustavo Anzueto Vielman of the Authentic National Center Party. ''The meeting represents a historic moment for Guatemala.''Many politicians, including Alvaro Arzu, Guatemala City's mayor-elect, still say the military junta must withdraw from the political arena after announcing new elections and leave politics to the politicians. ''We, as politicians, can't attempt to tell the Army how to run their barracks . . . ,'' Mr. Arzu told the Monitor. He refuses to take office as mayor if he will be a representative of the military junta and not the citizens who elected him.

The Army, however, remains the key to Rios Montt's survival at the head of the junta.

The junior officers, led by Capt. Carlos Rodolfo Munoz Pilona, made their move to oust General Lucas Garcia, analysts say, because they felt they were bearing the brunt of the guerrilla war while government officials became fatter on corruption. One observer says the Army foundered a bit after the coup but now ''is back in business.

'' General Rios Montt was brought out of retirement to advise the junta because of his reputation for honesty and discipline. The new government still faces a strong force of some 4,000 guerrillas concentrated in the country's western and northern regions. The guerrillas appear just as determined to fight General Rios Montt as the Lucas Garcia regime.

Although there was a brief lull in guerrilla attacks in the days following the coup, the guerrilla offensive appears to have intensified. One of the four rebel groups, the Guerrilla Army of the Poor, claimed April 25 that their forces had killed 116 government soldiers and shot down a helicopter during the past month. The report has not been confirmed.

The Army has reported numerous guerrilla incidents in villages in the Chimaltenango and Quiche regions, with many villagers killed by guerrillas searching for arms, food, and clothing. In the mountains, women are reportedly busy sewing uniforms for the guerrillas. When villagers report attacks by ''men in green uniforms,'' no one knows whether they are guerrillas or the Army. Most of the time the Army is blamed.

Partly as a result of this, the Army has begun a new - and apparently successful - strategy of arming some 30,000 peasants in villages vulnerable to guerrilla attacks. One of these civilian patrols recently reported having ambushed and killed three rebels in Quiche Province.

But there is lingering doubt about the Army's morale. One analyst with close ties to the military termed accurate early reports that some troops, loyal to General Lucas Garcia, resisted the military takeover in Quetzaltenango. At least 32 soldiers were killed in a two-hour firefight at the military base there, he says. The junta has dismissed these reports as a ''lack of understanding of the movement.

'' General Rios Montt still is very much respected by the junior officers from his days of teaching at the nation's military academy. But there is friction between the younger officers who led the coup and who are pushing the junta for reforms, and the senior officers who have, in effect, been stepped over. Rumors of new coups and countercoups keep everyone wondering.

The Army is keeping quiet for the moment and, according to one Western diplomat, it is even difficult for the diplomatic corps to gauge the mood of the young officers.

''I wanted to talk with some of the coupmakers at a party,'' the diplomat said. ''But the seating was arranged in such a manner that conversation with them was impossible. And I couldn't just go up to them and ask them to dance.''

Given the uncertainties within the Army, and the need to keep strong pressure on the guerrillas, analysts here feel Rios Montt could be pushed aside by the six junior officers if they feel he is moving too fast and dividing the Army.

If, on the other hand, General Rios Montt continues his strong campaign to capture public support and can produce concrete results, as one official suggests, his popularity may make his removal much more difficult in the long run.

He may even decide to call elections when he senses he is at the peak of popularity, and then offer himself as a candidate, this official adds.

But few officials are willing to predict what will happen here in the next few months, or how long General Rios Montt will stay in power. ''You have to remember this is Latin America,'' a Western diplomat says. ''It's not over yet.''