Liberia's military rulers, who took power three years ago in one of Africa's bloodiest coups, have begun to move this small, American-influenced West African country back toward constitutional civilian government.

Three weeks ago, the former master sergeant who led the coup, head-of-state Samuel K. Doe, accepted a draft constitution from a 25-member commission as the first concrete step in returning Liberia to an elected government by 1985.

If the transition is carried out as envisioned, Liberia would be the second country, after Nigeria, in black Africa's recent history in which the Army voluntarily stepped aside to make room for elected politicians. After several coups, a bitter civil war and 13 years of military government, Nigeria's civilian government took power in 1979.

During Redemption Day ceremonies here last week, Doe said the draft constitution "is currently being circulated throughout the nation so that all of our citizens will have an opportunity to carefully examine its provisions and make constructive inputs to that historic document."

The new constitution, which has been the subject of hearings and debates in 25 towns throughout the country for the past two years, still has to be considered by an elected national assembly before being put to a vote in a national referendum later this year.

In a country where most of the almost 2 million citizens have been afraid to discuss politics openly during the past three years, the acceptance of the draft by Doe has released a flood of unrestrained discussion here about Liberia's future.

But Doe created some fresh tension among potential office seekers when, in accepting the draft constitution, he announced that all government officials that plan to run for office when the present ban on politics is lifted must resign from office by the end of the month or be barred from running in any contests in 1985.

"Doe said that officials in his government were discouraging foreign investors" so that the military government would not get the credit for bringing industries to Liberia and the officials could not effectively perform their duties if they were looking forward to campaigning, said one high-level official who asked not to be identified.

"So people who have a desire to run for office are now afraid to declare themselves and resign because they would then stand out to the military government as troublemakers who were only pretending loyalty to Doe's government," he said.

Liberia's coup took place after a 133-year political and economic dominance of the country by an oligarchy made up of the descendants of freed American slaves, who had begun to emigrate here as early as 1822.

In 1847 the freedmen declared Liberia a self-governing republic. Known as Americo-Liberians, they ran the affairs of the country to the general exclusion of the more populous indigenous Africans.

That imbalance came to an abrupt end when Doe led a band of 16 noncommissioned officers and Army privates, all indigenous Liberians, in a fierce attack on Monrovia's executive mansion before dawn on April 12, 1980. When the shooting stopped, the government of president William R. Tolbert had been overthrown and Tolbert assassinated.

Ten days later, 13 members of the Americo-Liberian elite were publicly executed and their bodies riddled afterward with repeated gunshots in an outpouring of pent-up hate.

Doe's government suspended the old constitution--which made evident the nature of the government by beginning, "The love of liberty brought us here . . . "--and slapped an indefinite ban on all political activity. Those civilian politicians and local leftist activists who had brought matters to a head by agitating for change were brought into his government in a civilian cabinet. Many of these same former activists are today reluctant to resign.

The first two years were turbulent as rifle-toting soldiers roamed the streets settling old scores against the Americo-Liberians, indiscriminately confiscated property, harassed local businessmen and extorted money. An alleged palace countercoup plot ended in August 1981 with Doe ordering the execution of five of his closest associates in the coup the year before.

The rule of the gun ended by early 1982, when the government finally got the soldiers off the streets and Doe continued to consolidate his power. But in his Redemption Day speech, Doe indicated that he still faces problems today.

"While we are embarking on strong measures to improve the general financial condition in the country, we regret to note that many of our institutions are continually troubled by mismanagement, dishonesty and slothfulness," he said.

Doe also made it plain that his is still a military government, the draft constitutional debate notwithstanding. In an angry reference to a recent nurses' strike over working conditions that was supported by doctors, Doe said, "We must make it plain, fellow citizens, that as a military government, we will never submit to pressure from any source. This is the last time the government will allow any group to strike."

While Doe's government came to power with the promises of a revolution that would redress the long history of economic exploitation, Liberia's limited economic realities have tempered those intentions.

Already saddled with a large debt incurred partially by the Tolbert government to host the 1979 Organization of African Unity summit, Liberia has been hit badly by falling prices for its major commodity exports--iron, raw rubber and tropical woods. The repayment of the public side of its total $740 million debt had to be rescheduled recently, and Liberian officials say they will soon be forced to reschedule their commercial debts as well.

Liberia may default at the end of this month on part of a $60 million International Monetary Fund austerity assistance program because the government is $48 million over the agreement's $194 million ceiling on domestic borrowing from the Liberian central bank, according to several Liberian officials. At its present spending rate, Liberia could run up a $78 million budget deficit over the hoped-for $212 million in government revenues this year.

Doe has played down the ethnic animosity toward Americo-Liberians, and many who fled just after the coup have now filtered back under a general amnesty, without harassment.

"But an Americo-Liberian could never be president of Liberia again," said one well-informed Liberian official. "That day is now past."

Yet, other indigenous Liberians said Winston Tubman, minister of justice and a nephew of the still widely liked president William Tubman, an Americo-Liberian who died in office in 1971 after a long reign, is one Americo-Liberian who could win a presidential contest, although he says he is not a candidate.

There is also widespread speculation that Doe will be a candidate. But under the draft constititution a presidential candidate must be at least 35 years old, and Doe is only 30.

"I've talked to him and I don't think the head of state wants to succeed himself," said Assistant Foreign Minister Joe Morris.

Liberians, nonetheless, say they will be watching closely for any attack on the presidential age requirement, and the coming months here are expected to produce what may be the widest political debate Liberia has experienced.