It said the wiretaps of of the 73-strong EU observer team "reveal heavy suspicions that the results were rigged".
Gabonese intelligence wiretapped EU election observers who voiced grave doubts over the outcome of hotly disputed August 27 polls in the oil-rich central African nation, a French weekly reported.
In what it dubbed Gabon's "Watergate", the Journal du Dimanche (JDD) did not say how it had obtained excerpts of around 20 recordings, but said one of the subjects had "formally identified his own voice".
It said the wiretaps of some members of the 73-strong EU observer team "reveal heavy suspicions that the results were rigged".
The announcement that incumbent Ali Bongo won the vote with a razor-thin margin sparked two days of rioting and looting that left three dead in the former French colony, according to the government.
The opposition said dozens died in the unrest, during which some 800 people were arrested.
On one recording quoted by JDD, an unidentified EU observer is heard to say: "They are trying to work out how to cheat in a way that's not too obvious."
He adds: "Ballot boxes are on their way to (the capital) Libreville and will make the difference."
An EU spokeswoman said in a statement the observer team "had no knowledge it was being listened to."
Gabonese Communications Minister Alain-Claude Bilie Ny Nze dismissed the report Sunday as "trickery aimed at covering up the involvement of some European Union observers in favour of the opposition".
The EU mission was "neither neutral nor impartial," Bilie By Nze told AFP.
The EU spokeswoman dismissed the claim that the mission had not been neutral.
The mission "diligently respected (its agreement with the government and the electoral commission) in the exercise of its mandate, as well as the principles of neutrality and non-interference," she said.
The JDD report said the man in charge of security for the EU mission, named as Pierre B., was the "main target" of the wiretaps.
He is heard saying that there had been "changes to the numbers last night on Wikipedia", adding: "They increased the population of Haut-Ogooue. That's not encouraging."
The head of the EU mission, Bulgarian MEP Maryia Gabriel, told reporters on August 29 that the polls had been "managed in a way that lacked transparency".
The EU also said its election observers had had only limited access to witness the poll, in breach of the agreement the bloc signed with Gabon's government.
The next day, officials announced that Bongo had defeated challenger Jean Ping by fewer than 6,000 votes thanks to a 95 percent score in Haut-Ogooue, the southeastern fiefdom of the Bongo family.
They said turnout there was 99.93 percent.
"They did exactly what I hoped they wouldn't do," the deputy head of the EU observer mission is heard saying to Pierre B. in an exchange quoted by JDD.
Ping appealed the result to the Constitutional Court, which upheld Bongo's victory and put the winning margin higher at around 11,000 votes.
Bongo was officially sworn in to a second term last Tuesday, extending his family's rule in the country of 1.8 million people into a fifth decade.
A week ago, the EU mission said it "regretted" that the Constitutional Court "had been unable to satisfactorily rectify anomalies observed during the count".