He said he would prove that illicit money was used to buy votes and secure the victory of centrist candidate Enrique ...
He said he would prove that illicit money was used to buy votes and secure the victory of centrist candidate Enrique Pena Nieto, who denies this.
Mr Pena Nieto was confirmed the winner on Friday after a final recount, with 38.21% to Mr Lopez Obrador's 31.59%.
A protest march is planned in Mexico City on Saturday.
Six years ago, after losing the presidential election by a narrow margin, the left-wing Mr Lopez Obrador led weeks of protests that caused disruption in central areas of Mexico City.
Third-placed Josefina Vazquez Mota, from the right-wing National Action Party, has admitted defeat.
But the BBC's Will Grant in Mexico City says it is not just Mr Lopez Obrador's team who are pushing for alleged irregularities to be investigated.
There is a broad spread of people, not necessarily from the left, who feel that votes in their parts of Mexico were tampered with, he adds.
Accusations of irregularities led to a re-check of about 50% of votes after the result was announced on Monday.
The president of Mexico's electoral body said on Friday following the recount that there was no reason not to accept the result.
But Mr Lopez Obrador, of the Party of the Democratic Revolution, said the election had been fraudulent and that he would file an appeal next week.
He accuses the party of Mr Pena Nieto, the PRI, of paying for votes using gift cards for a supermarket chain.
Numerous videos have emerged of people claiming they received credit in exchange for voting for the PRI.
Mr Lopez Obrador promised to keep protests peaceful and "act responsibly".
Mr Pena Nieto, of the PRI party, told the BBC he had done nothing wrong and said he may sue his opponents over the allegations.
Our correspondent says the election had been a tortuous process, and that Mr Pena Nieto is unlikely to see a quiet acceptance of the result.
Mexico's Federal Electoral Institute has until early September to address complaints and formally announce a winner in the presidential election.
By BBC News