Nigerian President Olusegun Obasanjo has been sworn in for a second four-year term, marking the first time Africa's most-populous country returns a civilian government to power.

A dozen African leaders as well as representatives from more than 30 countries applauded as Mr. Obasanjo took the oath of office to begin a second and final term as Nigeria's elected president.

There was heavy security for the invitation-only ceremony in Abuja's Eagle Square, including a curtained pavilion of bullet-proof glass for the visiting dignitaries.

In his inauguration address, Mr. Obasanjo said he would draw on the lessons he has learned to build a great Nigeria.

The election marked the first time in Nigeria's history that a civilian government ran an election that was not blocked by a military takeover. But former military generals are at the center of the political process.

Mr. Obasanjo is himself a former military ruler from the 1970s. In this year's presidential race, his main challengers were three former army generals.

Turnout was high for the April 19 election. Though there was little violence, there were allegations of massive vote-rigging in the south and east. International and Nigerian observers expressed concerns about the conduct of the election, but none said Mr. Obasanjo's landslide victory was in doubt.

The main opposition party, the All Nigeria People's Party, is trying to overturn the results through the courts, claiming they are fraudulent. Opposition leaders have also called for mass protests against the new government. Before the inauguration, police blocked all attempts at demonstrations.

Mr. Obasanjo faces many other challenges during his second term, including tackling corruption more effectively, putting an end to recurring religious and ethnic violence and alleviating poverty.