The electoral authorities in Mexico have confirmed the winner of last weekend's presidential poll as Enrique Pena Nieto, of the centrist PRI.
His victory will restore the country's traditional party of power to office after a gap of 12 years.
However, the runner-up, Andres Manuel Lopez Obrador, is challenging both the result and the president-elect's legitimacy.
He said he would prove that illicit money was used to buy votes and secure the victory of Mr Pena Nieto, who denies this.
Mr Pena Nieto was confirmed the winner after a final recount, with 38.21% to Mr Lopez Obrador's 31.59%.
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 reports 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.