The fledgling civilian government of Africa's most populous nation has entered its third year faced with internal political tensions and potentially severe economic problems that threaten the future of civilian rule.

The man who has led Nigeria since its return to civilian control in 1979, President Shehu Shagari, is being challenged by a new, four-party political alliance crossing innumerable tribal, regional and religious lines that once were considered inviolable. They believe the president has ignored the problems of Nigeria's poor.

Coupled with the ever-widening economic chasm dividing the very poor from the very rich here, the political realignment has left many Nigerians apprehensive that the country is on the verge of the kind of climate that brought on the collapse of the first civilian government six years after independence, provoked a bloody civil war and resulted in 13 years of military rule.

Nigeria, which has one-fourth of Africa's entire population and the continent's largest oil deposits outside of North Africa, is envied, feared and looked to for leadership by the rest of black Africa. Many had hoped that Nigeria's return to civilian control would have a democratizing effect on this continent's numerous autocratic governments.

For the moment, however, Nigeria has abdicated its leadership role as it wrestles with its own political fate and prepares for what promises to be a tumultuous set of state and national elections in the summer of 1983.

The domestic unrest comes at a time when Nigeria's economy is extremely vulnerable because of the worldwide oil glut. Late last month the government slapped a freeze on virtually all imports to stem a dramatic decline in oil revenues that threatens Nigeria's ambitious multibillion-dollar development plan.

Shagari has recognized the problem publicly but expressed confidence. "The time is ripe to reflect on the vulnerability of our oil-based economy," he said in a recent speech. He concluded, "We have weathered the storm before and are determined and fully capable of doing so now."

Despite the oil shortfall, control of political office in Nigeria still means governing the distribution of billions of petrodollars that the country has earned since the mid-1970s. At the same time, critics say, corruption has skyrocketed along with a callous disregard by the affluent for the stagnant and potentially explosive living conditions of growing numbers of urban poor.

Neighboring Ghana's two recent military revolts, in 1979 and last Dec. 31, which pitted the affluent against the poor, were both greeted with trepidation by Nigeria's elite. The December coup, which borrowed its "revolutionary" ideas from Libyan leader Muammar Qaddafi, was followed here in February by the arrest of a businessman and seven Army officers in the first officially acknowledged military coup plot since Nigeria returned to civilian government.

Knowledgeable sources here agree that the plot never posed a serious threat. Indeed, the conspiracy appeared to involve only a handful of cynical soldiers financed by a disgruntled businessman who wanted the government overthrown because he had been cut off from the lucrative contracts he had won under military rule, the sources said.

Nonetheless, many saw the incident as a symptom of the manner in which questions of profit and loss have become part of the political fabric here.

"Factions are bitterly contending for supremacy and the customary restraints on the pursuit and use of power are being abandoned," said Nigerian Political Science Association President Claude Ake in a recent speech.

Because the government "owns" access to "status and wealth," the competition for state power has become a desperate struggle in which "politics becomes warfare, a matter of life and death," Ake added. "All over Nigeria, the economic discontent and anxiety are palpable."

Since last year, incidents of political violence, euphemistically called "bash-ups," have increased concomitantly with revelations of official corruption.

The violence has not reached anywhere near the levels of social anarchy and ethnic pogroms that precipitated the Army's takeover in 1966 and the tragic 2 1/2-year-long civil war that followed. But there has been periodic political violence in western Nigeria and a major riot last summer in the northern city of Kano, which ended in at least two deaths and the torching of more than $300 million in property.

So far, the violence has been contained, and the blatant manipulation by politicians of tribal hatreds--a prominent phenomenon in Nigeria's recent past--has not reappeared.

Instead, a four-party coalition has emerged of self-proclaimed "progressive" social reformers. Called the Progressive Parties' Alliance, the coalition boasts an explosive combination of conservative businessmen, socialist reformers and doctrinaire Marxist-Leninists, delicately held together by two old political enemies, Nnamdi Azikiwe, 77, and Obafemi Awolowo, 73.

The conservative Azikiwe was Nigeria's first president at independence. Populist Awolowo, leader of the opposition when Nigeria had a parliamentary-style government, lost to Shagari in the bitterly contested 1979 presidential election.

To avoid the "winner-take-all" politics that so characterized the early years of self-government, the Nigerians have created a federal-style government that borrows heavily from the U.S. Constitution.

The federal system has pushed Nigeria's politicians away from parochial politics, but it remains to be seen if such a disparate grouping as the alliance can evolve into a national movement. If it does hold together, some believe, the alliance may bring on a two-party system similar to that of the United States.

In Nigeria's Islamic north, where the majority of the country's 100 million inhabitants live, the alliance suffers from the perception of Awolowo--expected by many to be the group's 1983 presidential candidate--as an unreconstituted Yoruba tribalist. The Yoruba are one of Nigeria's three major ethnic groups, the other two being Hausa-Fulani and Ibo, who together compose 60 percent of the population.

The blunt-spoken Awolowo, who proudly bears the title of asiwaju, or leader of the Yorubas, said in a recently published interview that if that perception discourages potential supporters "then it is just too bad."

Shagari's supporters believe that northern Nigerians will not be able to ignore their visceral dislike for Awolowo's abrasive personality nor their fear that he will represent only Yoruba interests.

But new charges of corruption pose a major problem for Shagari's ruling National Party. In February, Ernest F. Allison, former deputy chief of Nigeria's embassy in Geneva, rocked the National Assembly with testimony that an official of a Swiss company doing business with the Nigerian government mistakenly had pushed an envelope stuffed with cash into his hand while they were at Geneva's airport seeing off visiting members of the assembly's House Finance Committee. When the official realized his error, he took back the envelope, which Allison said contained $120,000, and handed it to a committee staff member. Allison made his allegations to an assembly panel investigating alleged bribe-taking.

Such revelations have soured members of the National Party, said Dr. Ibrahim E. Ahmad, a founding member of the party.

"We felt we could create a true National Party for the first time," said Ahmad in an interview on his front porch in Kano. "But this was our undoing because the contractors, the '10 percenters' percentage of kickbacks to government officials on contracts , the crooks all rushed into the party also because they saw it as the winning party."

"To these people, contributions to the party are an investment, not a matter of national service to Nigeria," said lawyer Suleimanu Kumo, another National Party founder. "The crooks are too many and all they want to do is chop eat whatever they can get their hands on."

Both Ahmad and Kumo said that even while their party faces a formidable challenge from the Progressive Alliance, the state-level committees of the National Party are in turmoil and unable to plan election strategy.

Some party members believe the reformers are overstating the problem. "I think this business of chop-chop is crudely put," said Maccido Mohammed, a National Party member and deputy speaker of the assembly of Kaduna State. "Some of us are disillusioned, but the situation in Nigeria is no different from the rest of the world."

Shagari, who even opponents acknowledge has maintained a reputation for scrupulous honesty, has in recent months called for an "ethical revolution" to save Nigeria from the excesses of materialism and a return to ethnic politics.

"What one observes over the past decade," Shagari said in a January speech, "is that many, particularly among the elite, have become so materialistic that to them nothing matters except money and what money can buy."