Abe Sylla, a spokesman for 17 parties who fielded candidates in the race, said that "in Conakry, as well as in the interior of the country, there has been ballot-box stuffing".
The party of Cellou Dalein Diallo, the former prime minister, also alleged
irregularities, saying some ballot boxes containing 67,000 had "disappeared" and then been found by party supporters.
Alpha Conde, an opposition leader, said that in some locations, the electoral had set up polling stations dozens of kilometers away from where people had been registered to vote and then declared it illegal - for security reasons - for unauthorised persons to move around in vehicles.
However, Pathe Dieng, head of operations for the West African nation's independent electoral commission, denied the allegations and said there had been "no massive fraud'' in the vote.
Patricia Newton Moller, US ambassador to the country, said international observers had "found no evidence of widespread or systematic fraud'' and called political parties "to refrain from overstating electoral issues for political gain".
"Elections are never perfect,'' she said, adding that reports from international observers showed "that every effort was made to ensure that the process was fair and where issues arose, they do not appear to have been systematic, nor do they undermine the credibility of these polls.''
The US-based Carter Center, which sent observers to monitor the vote, said the electoral commission had "exhibited good faith efforts in their attempts to ensure a credible, transparent, and peaceful process".
Preliminary results were expected by Wednesday, but the Supreme Court allotted the electoral commission two more days to publish initial results.
Thierno Seydou Bayo, an electoral commission spokesman, said more time was needed to count all votes and the commission would not publish partial findings.
If no candidate wins a 50 per cent majority, the top two finishers will face off in a second round on July 18.
Sunday's vote had been praised as the first free vote since independence from France in 1958.
Guinea, which has one of the world's largest reserves of bauxite, the raw material used to make aluminum, and billions of dollars worth of iron ore, was a one-party state until its first coup in 1984.
The country was then governed for 24 years under the military rule of Lansana Conte.