Andres Manuel Lopez Obrador, the former mayor of Mexico City, said he has evidence for the fraud, including videos.
In one of them, shown to reporters on Monday, a man in the state of Guanajuato is seen stuffing several ballots into a box marked for the congressional race.
It was not clear who the votes were marked for, but Felipe Calderon of the ruling, conservative National Action Party won a razor-thin victory on July 2.
Lopez Obrador's supporters plan to visit foreign embassies in Mexico to ask that their governments do not congratulate Calderon until Mexican courts hear the appeals for a manual recount.
Calderon says the vote was clean and has taken congratulatory phone calls from the US president, George Bush, and the leaders of Canada, Spain and Colombia, among others.
A European Union team of observers said last week there was no serious fraud in the election.
US to heed recount
In a news release on Monday, Calderon's campaigners said they are looking at presenting their own evidence to the electoral tribunals, to back their claims that the election was fair.
Meanwhile, the White House spokesman, Tony Snow, said Bush would acknowledge any Mexican court rulings that could change who won the election.
"Should there be a recount, should there be another adjustment, should there be a change, then the president will acknowledge that," he said.
Lopez Obrador, who lost by less than one percentage point, led a rally of more than 100,000 people in Mexico City over the weekend and has called supporters out on to the streets in protests across the country later this week.
Manuel Camacho Solis, one of Lopez Obrador's campaign advisers, said that charges for political crimes brought by a special prosecutor days before the elections against a former president were part of a plan to pressure the ex-leader's Institutional Revolutionary Party, or PRI, to support Calderon.
"They went to the extreme of ordering house arrest for ex-president Luis Echeverria a few days before the election on genocide, only to drop them two or three days after the election," Camacho Solis said.
Lopez Obrador's main hope lies in electoral tribunal appeals that allege there were nationwide violations before the election, including campaign overspending, government support for Calderon, and unfair intervention on his behalf by business and church groups.
More specific appeals are being filed in at least 151 - and perhaps all - of the country's 300 electoral districts, claiming mainly that ballots are either unaccounted for, or that there were more votes than ballots or registered voters, at specific polling places.