Nigeria's President Goodluck Jonathan has ended months of speculation and confirmed he will contest January's elections.
He made the announcement on his Facebook site, saying the decision had been taken "after wide and thorough consultations".
The president's Facebook statement said he would make a formal declaration of his intention to stand for election on Saturday.
Officials in the presidency have confirmed that the Facebook entry was genuine.
While some observers were surprised that Mr Jonathan had used Facebook to announce his candidacy, he said in July that comments on his page had influenced him in overturning his ban on the national football team.
He announced the ban after the Super Eagles' poor showing in the World Cup finals.
"People may scoff, but we take the interactions seriously, we track the [Facebook] feedback," a presidential adviser told the Reuters news agency.
"It's a small platform perhaps, but it offers the possibility of change."
Only a small fraction of Nigeria's 150 million people have access to computers, but numbers are growing fast.
The announcement came as one of Mr Jonathan's main rivals for the governing People's Democratic Party's nomination, former military leader
Gen Ibrahim Babangida, launched his campaign.
Goodluck Jonathan's declaration finally answers the question that has obsessed Nigeria - whether he would run, or not.
For some in the political elite, it is highly controversial. For them, Mr Jonathan's southern heritage is an obstacle - they believe that the next president should be from the north.
Now it is clear he will run, he faces an intense struggle in his own party. Some of the wealthiest - and most adept - manipulators in politics are ranged against him, and will fight him for his party's nomination.
His rivals include former military leader Gen Ibrahim Babangida, former Vice-President Atiku Abubakar, Kwara state governor Bukola Saraki and national security adviser Aliyu Gusau.
It promises to be a tough and volatile election, and the struggle could tear the governing PDP apart.
Thousands of people gathered for his rally in the capital, Abuja.
Several other heavyweight politicians are expected to seek the PDP ticket, which could be divided along north-south lines.
The PDP has a tradition of alternating power between northerners and southerners for two terms each.
Under this unwritten rule, its candidate in 2011 should be from the largely Muslim north, rather than the mainly Christian and animist south, which would favour Gen Babangida or former vice-president Atiku Abubakar rather than Mr Jonathan.
The BBC's Caroline Duffield in Lagos says it is clear party bosses have privately agreed that - because of the exceptional circumstances in which Mr Jonathan inherited power - he should be allowed to run.
He is the first president from Nigeria's southern, oil-producing Delta region.
But his statement said he had held consultations across the country before deciding to run.
Nigeria's recent elections have been tarnished by fraud and violence.
Mr Jonathan has promised to introduce electoral reforms, but correspondents say it will be difficult to implement radical changes before January.
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