Tension is high in Guinea with some streets reportedly deserted as security forces patrol them to contain a post election violence.
Supporters of the opposition have been clashing with security forces after Guinea’s President Alpha Condé won a third term in office.
Preliminary results from the electoral commission showed that the 82-year-old secured 59.5% of the vote.
Mr Diallo, 68, won 33.5% of the ballots, the electoral commission said.
His victory will now have to be confirmed by the Constitutional Court but his opponent, Cellou Dalein Diallo alleges large-scale fraud.
So far at least 30 people have died since the elections were held amid violent reactions to results.
A delegation comprising officials from the United Nations, the African Union and regional bloc ECOWAS is expected in Conakry to resolve the crisis.
“We are still going to refer the matter to the constitutional court, without having too many expectations,” Mr Diallo told Agence France-Presse.
He added that “We are going to protest against this electoral hold-up through the street. We are in the process of building a case, which is very difficult since our office is occupied, we don’t have access to our documents, but we are nevertheless going to refer the matter to the constitutional court.”
Meanwhile the ruling party has asked the opposition to desist from violence amid the declaration of results.
“My call to the opposition activists: lay down weapons, be calm and look forward to the future. Nothing is worth destroying Guinea,” remarked Sékou Condé, permanent Secretary of the ruling RPG party.