Millions of Nigerians were awaiting the results of a formal challenge seeking to overturn the apparent election as president of Shehu Shagari, who is scheduled to take over from the military government on Oct. 1 after 13 years of Army rule.

Shagari, 55, leader of the National Party, placed first in a five-way presidential race on Aug. 11. The runner-up, Obafemi Awolowo, 70, leader of the Unity Party, has challenged the designation of Shagari as president on the basis of a disputed interpretation of the country's newly adopted constitution.

Nigeria, the most populous country in Africa and a major world producer of oil, has been under military rule since 1966. Nigeria's population, estimated at up to 100 million, has been through a bitter civil war, precipitated by a political crisis in its first civilian government after independence from Britain in 1960. It has since seen three military coups and an attempted coup.

The current military head of state, Gen. Olusegun Obasanjo, 42, began the return to civilian rule last September by lifting a 13-year ban on political activity.

Since July, there have been elections for an American-style Senate, House of Representatives, 19 state legislatures, 19 state governors and, lastly, the presidential election just over two weeks ago.

The earlier elections generated only minor disputes but the presidential election has triggered great controversy, according to reports reaching here.

On Tuesday, an election tribunal is to begin hearing Awolowo's petition to nullify Shagari's election.

Awolowo's Unity Party has formed a three-party coalition with Nnamdi Azikiwe's Nigerian People's Party and Waziri Ibrahim's Great Nigerian People's Party to challenge Shagari's National Party in the legislature. Although Shagari's party has the most seats, it does not control either house.

A fifth presidential candidate, Aminu Kano, who won a last-minute court reprieve to run after being disqualified for nonpayment of taxes, has denounced the coalition and called for a five-party national government. He is leader of the People's Redemption Party.

Central to the dispute is a passage of the constitution requiring that a president win at least 25 percent of the votes in two-thirds of the country's 19 states.

Shagari received 5,688,857 votes, with 25 percent or better in 12 states. In the northern state of Akno, he won only 20 percent and therein lies the crux of the present crisis.

Gingerly attempting to step around a confrontation, Nigeria's Federal Electoral Commission ruled that he had substantially met the requirement and had won. But Awolowo, a distant second with 4,916,651 votes, disagreed.

A ruling will be made by the electoral tribunal next week.

In the meantime, Adamu Ciroma, secretary general of Shagari's National Party, has rejected the People's Redemption Party call for a national five-party government.

The National Party, Ciroma said, will take over executive powers from the military without sharing ministerial portfolios with leaders of the other four losing parties.