Six weeks after a palace coup, Gen. Oscar Mejia Victores has yet to consolidate his power in this most populous Central American nation, according to well-informed Guatemalans and western diplomats.

Mejia's failure to establish unquestioned leadership threatens to create new problems for the Reagan administration in this troubled region. Guatemala, like El Salvador to the south, is fighting left-wing guerrilla revolutionaries.

Mejia's problems, according to local and foreign analysts, are the result of his personal limitations as a leader as well as a fragmentation of the officer corps that has dominated politics here since 1954--when a CIA-sponsored coup overthrew a freely elected leftist government.

When Mejia seized power Aug. 8, his move was seen as a restoration of the armed forces' traditional chain of command. It had been disrupted in March 1982 when younger officers, upset by the alleged corruption and ineffectiveness of their elders, overthrew general Fernando Romeo Lucas Garcia.

The young officers installed as head of state the former commander of their military academy, retired general Efrain Rios Montt, a fundamentalist convert who set out to reform this nation of 7 million people with the word of God.

One reason for Rios Montt's demise was his refusal to specify when he would leave office, although he often referred to eventual elections. Mejia, who continues in the defense minister post from which he launched the coup against Rios Montt, has stressed that his rule is transitional. He has called for election of a constituent assembly in July and has not assumed the title of president. According to the same sources, he has not exercised the powers traditionally concentrated in that office here.

Mejia, 52, was little known outside of his own military circles. He is thought to have been chosen to replace Rios Montt mainly because he was the senior general on active duty.

That it was the senior commanders' intent to reimpose a rule of seniority in the military was underlined by Mejia in his first press conference in August. "There is no such thing as young officers," he said, "only subaltern, superior and general officers."

Not only has Mejia's new order produced few substantive changes in domestic or foreign policies but, in the words of a senior Western diplomat here, his actions to date have made him "appear indecisive and inept."

Mejia initially lifted the modified martial law imposed by Rios Montt and abolished his predecessor's secret tribunals that had sent 15 men to their deaths without public trial or judicial review.

He spoke of ushering in a new era of cooperation with Washington that would see resumption of military aid. It was suspended by then-president Jimmy Carter in 1977 because of Guatemala's human rights record.

Known to his critics as a "man of the machete"--a Guatemalan euphemism for a military hard-liner--Mejia had a sharp exchange earlier this year with Rep. Clarence Long (D-Md.) on the issue of Army excesses against civilians. According to U.S. officials, Mejia's talk of resumed U.S. aid lacked any realistic basis.

Similarly, Mejia promised to reduce an unpopular 10 percent value-added tax imposed by Rios Montt. Instead, he backed down and has promised a "government review" of the tax.

More alarming to western analysts than questions about Mejia's leadership capacity is evidence that the armed forces are divided about who should lead the nation in the long run, and how.

"Someone I trust told me there is the equivalent of a golpe coup every day among the officer corps," said a European ambassador. "No one knows who the real power is in the Army these days."

According to analysts, one of the most important factions is that led by Gen. Mario Lopez Fuentes, the armed forces' chief of staff who is widely considered the power behind Mejia. An ambitious officer with ties to the extreme rightist National Liberation Movement of Mario Sandoval Alarcon, Gen. Lopez enhanced his image with Catholic Guatemalans when newspapers ran photos of him being received by Pope John Paul II at the Vatican last week.

These analysts list Col. Hector Gramajo, the deputy chief of staff, as a counterweight to Lopez. The colonel's power is said to rest on his close contacts with regional commanders, whom he oversees.

The third locus of military power, according to those with contacts in the Army, remains the young officer corps. It is reliably reported that these men had been planning their own coup against Rios Montt when Mejia moved.

Although the most prominent young officers who had advised Rios Montt in the presidential palace are under loose house arrest, they reportedly remain a powerful movement within the Army and are little disposed to return to barracks permanently.

Yet a fourth important faction of old guard generals has begun to coalesce around a former chief of staff, general Benedicto Lucas Garcia, who has been living in forced retirement on his farm since the March 1982 coup overthrew his brother.

The military's search-and-destroy tactics that have thrown leftist guerrillas off guard are attributed to the general and there has been speculation that he might be put back on active duty in an important post.

Such behind-the-scenes jockeying for power within the Army has convinced many observers that Mejia's rule will indeed be transitory.

U.S. officials hopeful of seeing stability in Guatemala privately express disappointment at developments here. They pin their hopes on Mejia's promises to hold elections that could lead to some form of constitutional rule. These officials express the hope that before Mejia is ousted, the mechanisms for such elections might have been set in motion.

They were encouraged last week when Mejia, in an Independence Day speech to the nation, promised to hold elections for a constituent assembly by next July. The resultant constitution would pave the way for a transition to civilian rule.

"Our policy here is simply that we do not want to see a revolving door, with one general succeeding another," said a senior U.S. official here. "We think the only long-term hope for Guatemalan stability is for it to return as quickly as possible to the sort of constitutional rule that will give the government some legitimacy, so that we can give it support. I think Mejia realizes he has to have elections or he will be thrown out," the official said. "It is thus in his interest to have them soon rather than later."

Such a scenario is not universally accepted. Many in the military--especially the old guard with vested interests in the traditional form of military government--would want elections only if they would bring to power their right-wing civilian allies--men like Sandoval Alarcon who have called for a reign of terror against subversives to purify the body politic.

"I don't think Mejia will remain in power long enough to see any elections held," said a European diplomat. "I think he will stay in power until the elections are set, then there will be another military golpe that will prevent them being held. That, after all, has been the history of Guatemalan politics in our time."