Unofficial results from Senegal's presidential election suggest a tight race between incumbent Abdoulaye Wade and former Prime Minister Macky Sall.
A source in the electoral commission has told the BBC that a run-off between the pair is "likely", backing local media reports and Mr Sall's comments.
President Wade, 85, is seeking a third term in office despite serving a constitutional limit of two terms.
Mr Wade was booed as he cast his vote on Sunday in the capital, Dakar.
They could be heard shouting: "Get out, old man!"
Mr Wade's decision to stand again has sparked weeks of violent protests - leading to about six deaths - although polling day itself was largely peaceful.
Early reports indicate that Mr Wade and Mr Sall have each won between 20-35% of votes - more than any of the other 12 candidates.
The electoral commission has yet to publish any provisional results, which are being announced by local media as they trickle in from polling stations.
The commission says it will publish its first results on Tuesday.
Mr Wade lost in his own constituency in the middle-class Dakar neighbourhood of Point E, Senegal's national APS news agency reports.
'Anyone but Wade'
Mr Wade's spokesman Serigne Mbacke Ndiaye told the BBC he remained confident of a first round victory as the president is "well ahead".
But Mr Sall believes that "a second round is inevitable, we have won the biggest departments in the country."
The 50-year-old geologist and mayor of the western town of Fatick also warned against rigging.
Mr Sall, who is running for the first time, told the BBC that it would be "easy" to win in the second round, as he expected all opposition supporters to unite behind him.
The BBC's Thomas Fessy in Dakar says Mr Sall has been the only opposition candidate to run a real electoral campaign around the country, as other leaders struggled to mobilise people in street protests.
If a run-off is confirmed, an "anyone but Wade" coalition could emerge behind Mr Sall, our correspondent says.
Senegal's constitutional court ruled that Mr Wade could stand again on the grounds that his first term had not counted since it began before the two-term limit was introduced in 2001.
The court also barred world-famous singer Youssou N'Dour from standing in the election.
Mr N'Dour has argued that allowing Mr Wade to run again amounts to a constitutional coup d'etat.
Senegal, a former French colony, is seen as a stable democracy with an unbroken series of elections since independence in 1960.
It remains the only West African country where the army has never seized power.
'Father of the nation'
A cacophony of boos and jeers drowned out clapping from a few dozen supporters when Mr Wade appeared at the polling station in the Dakar suburb of Point E, where he owns a private home, AFP news agency reports.
Visibly angry, the elderly leader reportedly pushed aside one of his own bodyguards as he beat a swift retreat after voting, without speaking to the media.
Mr Wade is one of 14 candidates who also include Mr Sall and Idrissa Seck, both former prime ministers who served under him.
However, he is confident of victory, insisting his advanced age is an asset.
"I am president and father of the nation. This is what the Europeans do not understand," he told French weekly Sunday newspaper Le Journal du Dimanche.
Most opposition candidates have called on their supporters to go to the polls and vote Mr Wade out of power but they have also insisted they will not recognise his victory if he is re-elected, our correspondent says.
Speaking at a polling station in Dakar's Amitie district, N'Dour said: "The Senegalese are not stupid... The rest of the world is watching what goes on in Senegal very closely."
International leaders appealed for calm ahead of the vote.
The African Union's envoy, former Nigerian President Olusegun Obasanjo, proposed that Mr Wade retire after two years if re-elected.
But his plan was not accepted by either the president or the opposition.
Once a veteran opposition leader himself, Mr Wade was first elected in 2000 - ending 40 years of rule by the Socialist Party.