"I will be the president of change for everyone’s benefit, I will be the president of national reconciliation, of progress for everybody," said Conde after the announcement.
"The light of democracy is on the horizon. We have to seize this opportunity to make it a tangible reality for the good of our people," he added.
"It was total joy in Conakry last night especially among supporters of Alpha Conde’s party," said Conakry correspondent Karim Kamara.
Ceni reported a 67 per cent turnout in the second round. Diallo has insisted that he will not accept the results until there is an investigation of “massive fraud” at all levels.
"Cellou Dalein Diallo is still insisting that he will not accept the election result until ballots in three towns in Upper Guinea, a stronghold of Alpha Conde, are annulled" Kamara told RFI.
The Supreme Court has yet to ratify the results. Diallo said he would prove to Guinea’s highest court that the vote was “riddled with irregularities and fraud”.
Conde offered his "sincere" congratulations to Diallo and said that he would "extend to him the hand of fraternity for a united and prosperous Guinea".
The result is a big turnaround for Conde who took 18 per cent in the first round as Diallo secured 43 per cent.
"Shortly after the announcement of the result security forces began firing into the air forcing people to go to bed until this morning," said Kamara.
"The measure was aimed at supporters of Cellou Dalein Diallo’s party who were planning to carry out another street protest like they did yesterday morning," he added.
Diallo called on his supporters to “remain calm, avoid provocation and violence of any nature” following clashes on Monday that left one person dead and several injured. Guinean soldiers shot and killed a man in Dalaba, in Mamou region, according to witnesses quoted by the AFP news agency.