Nigeria's acting president, Goodluck Jonathan, took the oath of office Thursday morning, hours after Africa's most populous nation woke up to news its elected leader had died.
Jonathan will serve as president until the next election, expected to be held next year.
President Umaru Yar'Adua died Wednesday after battling a heart ailment that had kept him out of the public eye since November. A state funeral will be held in his native Katsina, followed by seven days of national mourning.
The 58-year-old leader went to Saudi Arabia for treatment in November, creating a power vacuum in the West African nation. He was treated for an inflammation of tissue around his heart and returned to Nigeria in February.
However, he remained out of sight, and Jonathan has served as the country's acting leader since his return.
Yar'Adua took office in 2007 in an election mired in controversy and accusations of vote-rigging.
"There was ballot snatching, voters were molested, voters were beaten ... and also payment inducement to vote for certain candidates," said Eneruvie Enakoko of the Civil Liberties Organization, a human rights group in Lagos.
Despite the accusations, the president - a soft-spoken and unassuming figure who did not bask in the media spotlight like past leaders of the West African nation - pledged to fight to improve the country of 150 million people.
"Our collective goal is to deliver for our children a Nigeria better, stronger, more peaceful, more secure and more prosperous than we met it," Yar'Adua said.
U.S. President Barack Obama and U.N. chief Ban Ki-moon expressed their condolences over Yar'Adua's death.
They applauded his quest for stability, especially in Nigeria's oil-rich Niger Delta, where he offered amnesty to rebels battling government forces.