Mr Lopez Obrador rejected the result, accusing Mr Pena Nieto's Institutional Revolutionary Party, or PRI, of buyingvotes and favourable media coverage.
Opponents of the PRI said they have video and photo evidence of the party buyingvotes through thousands of cards that could be redeemed for products at a chain of supermarkets.
He said Congress must stop its old game of promising endless subsidies in the hope of buyingvotes, and instead concentrate on reforms that would induce rapid growth, the best antidote to poverty.