President Umaru Musa Yar'Adua, 58, has died after a long illness, the Nigerian government has announced.
Chief press secretary to the president, Segun Adeniyi said he died at 9pm Nigerian time on Wednesday. The President had been unwell and had not been seen in public since October last year.
Acting President Goodluck Jonathan has visited the first family and offered his condolences. World leaders led by US President Barack Obama have also joined in mourning the Nigerian leader.
"Nigeria has lost the jewel on its crown," Jonathan said, announcing seven days of national mourning.
Yar'Adua will be buried in his northern home town of Katsina at 2 p.m. (9 a.m. ET) on Thursday. The government has declared Thursday a national holiday and the national flag will fly at at half mast.
President Yar'Adua's death sets the stage for the most hotly contested succession since the country's return to democracy a decade ago.
Acting President Jonathan - who has been running Africa's most populous nation for months during Yar'Adua's illness - is expected to be sworn in as head of state and appoint a new deputy, according to the constitution.
The pair will then complete the unexpired presidential term in the oil-producing nation of more than 140 million people until elections due by April 2011.
President Obama said his thoughts and prayers were with Yar'Adua's family and remembered his "profound personal decency and integrity."
Yar'Adua had been absent from the political scene since November, when he left for medical treatment for a heart condition in Saudi Arabia. He returned to Nigeria in February but remained too sick to govern.
Jonathan assumed executive powers in February and has since consolidated his hold on power, appointing a new cabinet and his own team of advisers. But Yar'Adua's death raises the stakes in the run-up to the next elections.
It is unclear if Jonathan, who is from the southern Niger Delta, will run for president because of an unwritten agreement in the ruling party that power rotates between north and south.
The next four-year term is due to go to Yar'Adua's Muslim north.
"The paramount issue will be who the new vice president will be. It'll probably be a northerner (who) will be front runner for the presidency in 2011," said Kayode Akindele, a director at Lagos-based consultancy Greengate Strategic Partners.
A presidential aide told Reuters there was no rush to swear in Jonathan as head of state because, as Acting President, he is already commander-in-chief.
Yar'Adua, who pledged respect for the rule of law when he took office, initially was seen by many Nigerians as a breath of fresh air after eight years of former president Olusegun Obasanjo, an overbearing ex-military ruler with a penchant for disregarding court orders and legal detail.
He was Nigeria's first university-educated leader and won victory in April 2007 polls which, though marred by intimidation and ballot-stuffing, marked the first transfer of power from one civilian president to another since independence in 1960.
But the optimism quickly faded.
Yar'Adua earned the nickname "Baba Go-Slow," a reference to the local term for Nigeria's crippling traffic jams, for what critics said was slow progress on everything from economic reforms to restoring the shambolic energy sector.
His biggest achievement was in the restive Niger Delta, the heartland of Africa's biggest oil and gas industry.
Militant attacks rumbled on during the early part of his tenure, but his offer of amnesty last year led thousands of gunmen to lay down their weapons and has brought more than six months of relative peace in the region.
The main militant group in the region, the Movement for the Emancipation of the Niger Delta (MEND), said it was saddened by Yar'Adua's death.
"MEND considers the late president a genuine peacemaker whose initiatives, humility and respect began to bring confidence to the peace process," the group said in an email to Reuters.
"His death may leave a vacuum that may not be filled."
(Additional reporting by Camillus Eboh and Chijioke Ohuocha; Writing by Nick Tattersall; Editing by Michael Roddy)