Mr Sanha (L) and Mr Yala have both promised peace and stability
Elections in Guinea-Bissau to replace assassinated President Joao Bernardo Vieira are to go to a second round, the electoral commissioner says.
Mr Vieira was killed in March in apparent revenge for the death of the head of the army in a bomb blast.
The two frontrunners from Sunday's vote are the ruling party's Malam Bacai Sanha with 39.6% of the vote and former President Kumba Yala with 29.4%.
They will face each other in a run-off - the date of which is yet to be set.
Eleven candidates were participating in what the head of the European Union's observer mission Johan van Ecke said was a well-organised election, Reuters news agency reports.
"The vote took place in a calm and orderly fashion, and voters were able to exercise their right freely," he said.
The voter turnout was 60%, AP news agency reported.
Both Mr Sanha and Mr Yala promise to bring peace and stability to the impoverished West African nation.
They have also promised to tackle the drug trade in Guinea-Bissau, which has become a transit point for gangs shipping cocaine from South America to Europe.
Mr Vieira had ruled Guinea Bissau for 23 years, from 1980 to 1999 and from 2005 to 2009.