LAGOS, Nigeria - President Olusegun Obasanjo took a commanding lead in early returns from voting that could bring Nigeria's first successful transition from one civilian government to another, but the main opposition party claimed the vote was rigged and a "huge joke."

With more than 20 million votes counted in 21 of Nigeria's 36 states and the federal capital territory, Obasanjo had 66 percent of the votes compared with 27 percent for Muhammadu Buhari, a former ally.

Buhari's campaign spokesman, Sam Nda-Isai, said his party rejected the results.

"The entire so-called election is a huge joke," Nda-Isai told The Associated Press. "As far as we are concerned, democracy has failed."

Buhari and other party leaders will meet in the next few days to decide a course of action, the spokesman said, declining to speculate whether that could involve massive protests as opposition officials earlier warned.

Obasanjo's campaign spokesman, meanwhile, called the results "a victory for the entire country."

"The people have spoken," Akin Osuntokun said, dismissing opposition threats as "the inability of losers to accept defeat gracefully."

Nigeria's election commission promised to cancel results in areas with proven cases of fraud.

"As far as we are concerned, (so far) there has been no rigging," Abel Guobadia, the election commission chairman, said Sunday.

The election was a major test of whether democracy has taken root since Obasanjo was elected four years ago, ending 15 years of brutal military rule.

Nigeria - Africa's most populous nation - has never seen a civilian government successfully hand over power to another.

Though it is one of the world's largest oil exporters, it is desperately poor and has a history of coups and unrest.

Obasanjo was once a military ruler, but he traded his uniform for traditional robes and ran for election in 1999. In Saturday's vote, he faced 19 challengers, including Buhari. The two former army men were once close colleagues, but their relationship soured after Obasanjo accused Buhari of mismanagement.

Police and soldiers patrolled some cities, fearing unrest. In the central town of Bukuru, police fired tear gas and bullets in the air to disperse hundreds of Buhari supporters marching through the streets carrying tree branches - a common West African protest symbol. Demonstrators accused an incumbent governor of using fraud to win a second term. No results have been announced there so far.

The vote was marred by a shooting at a polling booth in the oil-producing Niger Delta, the scene of a month of ethnic and political violence that has left more than 100 people dead. Six were killed in the attack, election monitors said.

Nearly half of Nigeria's 126 million people registered for the ballot in 36 states and the capital. It was unclear how many voted, though officials said turnout was strong.

"Where there is substantial violence and boxes disappear, the results will not be accepted," stressed Hakeem Baba-Ahmed, commission secretary, who declared the ballot a "vast improvement" over previous elections.

At home in the southwest, the incumbent president smiled but declined to speak to reporters. A group of village women outside his house cheered when he gave them a bundle of cash - a common custom in Nigerian politics. Obasanjo was preparing a speech to the nation to deliver Tuesday, his spokesman said.

Obasanjo's rule has brought some improvement in individual and press freedoms. But outbreaks of political, religious and ethnic violence have left more than 10,000 people dead since 1999. The leader's critics say he has done little to fight poverty and corruption.

Buhari gained notoriety by launching a 1983 coup that toppled civilian leader Shehu Shagari after elections widely regarded as flawed.

Neither candidate has articulated a platform. Their campaigns have largely drawn upon Nigeria's religious and ethnic fault lines. Obasanjo is a Christian from the Yoruba ethnic group, with a strong support base in the southwest. Buhari is a Fulani and a Muslim from the north.

Buhari was once a trusted Cabinet minister in Obasanjo's military regime of the 1970s. Yet their friendship soured in 1999 when Obasanjo dissolved a government trust fund Buhari headed, accusing him of mismanaging funds.

Buhari was further alienated when Obasanjo later ordered the country's military leaders to testify to a commission investigating junta-era crimes.

In many parts of the Niger Delta, a boycott and allegations of fraud prevented voting from taking place. Journalists and observers witnessed apparent corruption, including ballot box theft and bribery.

One monitoring group, the International Republican Institute, described the elections as largely peaceful but said results were "significantly flawed" in three of 12 states where it had observers.

"Ballot tabulation and collation were highly irregular and created the opportunity for abuse" in those three southeast states, the Washington-based group said.

PHOTO CAPTION

Current Nigerian President Olusegun Obasanjo of the Peoples Democratic Party (PDP) will await the election results Sunday, April 20, 2003. Obasanjo took a commanding lead in elections seen as a key test of Nigeria's young democracy, according to early results, but the opposition accused his ruling party of ``massive'' vote-rigging and warned that people would fight the results. (AP Photo/Ben Curtis)