ABUJA, Nigeria (AP) – A reclusive former governor from Nigeria’s Muslim north was sworn in as president Tuesday, in the first transfer of power between civilian leaders in Africa’s most populous country.

Umaru Yar’Adua used his inaugural speech to call for an immediate end of hostilities in the southern oil region, a conflict that has cut output by nearly a third in Africa’s biggest petroleum producer. Oil prices fell Tuesday as the main militant group in the region said it was considering the request, reflecting hopes that the inauguration of a new president in Nigeria would contribute to stability in the market.

While Mr. Yar’Adua’s inauguration marked a milestone in a country struggling to consolidate democracy after decades of military rule, he was elected in an April vote the opposition has denounced as fraudulent and international observers have said was not credible.

Mr. Yar’Adua, 56, addressed the voting controversy head on, acknowledging at the start of his speech that the elections were flawed. He promised to set up a panel to examine the electoral system “with a view to effect reforms.”

“We acknowledge that our elections were not perfect and had lapses and shortcomings. However, we have well established legal avenues of redress and I urge anyone aggrieved to pursue them,” he said to applause.

The violence in the Niger Delta oil region poses one of the biggest challenges facing Yar’Adua. Militants have staged a series of attacks on the industry, demanding the federal government designate more oil resources to their impoverished region.

Former President Olusegun Obasanjo had dismissed the militants as mere criminals and made no mention of them in his farewell speech late Monday.

In contrast, Mr. Yar’Adua said the matter needed “urgent attention” and said he would “use every resource available to resolve this crisis.”

A spokesman for the Movement for the Emancipation of the Niger Delta told The Associated Press in an e-mail the group was considering Yar’Adua’s call for an end to the violence. “We are considering this request,” he said.

The group had earlier rejected all calls to lay down arms until their demands were met.

The global oil industry hopes “that the new administration will manage to bring some stability to the country,” Vienna’s PVM Oil Associates said.

Earlier Tuesday, troops and police set up roadblocks and patrolled streets in volatile districts of Nigeria’s biggest city, Lagos, saying opposition leaders would not be allowed to go ahead with a planned protest of the inauguration. Mr. Obasanjo’s 1999 election ended decades of near-constant military rule, including a particularly brutal 15-year phase that began in 1984. As the head of the ruling party, he is expected to continue playing a top role in the country.

Mr. Yar’Adua said his inauguration was a “historic milestone” for his restive nation of 250 ethnic groups and 140 million people, almost equally split between northern-based Muslims and southern Christians.

“We have at last managed an orderly transition from one elected government to another,” he said. All other attempts since Nigeria’s 1960 independence from Britain were undermined by coups d’etat or annulments of electoral outcomes.

While the end of repressive rule unleashed long-simmering conflict that has left 15,000 dead in political violence, Nigerians are more unified, said Mr. Yar’Adua.

Under civilian rule, Nigerians say they have gained freedoms and their country has shaken its reputation as an international pariah run by generals bent on looting the public coffers. Obasanjo has cleared the country’s books of billions of dollars of debt wracked up by the military rulers and helped end at least two of West Africa’s civil wars by sending troops to intervene.

But many Nigerians say graft has continued to flourish under Obasanjo, and services like electricity and water supply have degraded. The vast majority of people in Africa’s biggest oil producer live below the poverty line, while a largely corrupt political and business elite linked to Obasanjo has grown.

Mr. Yar’Adua said he would build roads, power-generation plants and transfer infrastructure, fight corruption, create jobs and keep inflation and interest rates low to attract foreign investment.

“Let us now join together to build a society worthy of our children,” he said.