The runner-up in Mexico's presidential election has refused to accept its result, claiming that his rival Pena Nieto swept to victory on the back of an organised campaign of fraud, bribery and vote-rigging.
Lopez Obrador – who with 99 per cent of ballots counted trails by 3.2 million votes – claimed Mr Nieto had illegally bought at least a mi llion votes and exceeded the campaign finance limit. He pledged to challenge Sunday's election result in court, but stopped short of calling supporters on to the streets, as he did after narrowly losing to the outgoing President Felipe Calderon in 2006.
"I cannot accept any results, until I have complete certainty that the citizens' vote was respected, and the election was not falsified," he told reporters, adding that Mexicans who had backed Mr Nieto were voting for a "regime of corruption."
Mr Nieto, who has already claimed victory, represents the right-leaning Institutional Revolutionary Party (PRI), which used patronage and bribery to hold power in Mexico for more than 70 years, before losing a presidential election in 2000.
Mr Obrador, a left-wing former mayor of Mexico City, meanwhile has a track record of refusing to accept election defeats. In 2006, when he lost by one per cent, he claimed fraud and organised strikes and mass protests which paralysed the country's capital for more than a month. At one point, he even swaggered into the Zocalo, Mexico City's central square, and staged a "swearing in" ceremony to declare himself President.
Some believe that affair cost Mr Obrador the support of serious-minded voters. This time round, his defeat was certainly more comprehensive: he currently boasts just 31 per cent of the vote, compared with Mr Nieto's 38 per cent.
There are also questions over the ability of his Party of the Democratic Revolution (PRD) to organise a substantial protest. Last year, when the PRD lost state elections by 41 points, they also cried fraud, and called supporters on to the streets. But turnout was poor and the campaign fizzled.
While most analysts believe that there was at least some foul play in Sunday's election – and there are growing questions over the means by which Mr Nieto came to secure vociferous support from Mexico's mainstream media – it seems unlikely that the scale of the fraud was wide enough to have affected the result.
The PRI has portrayed Mr Obrador's failure to accept defeat as a case of sour grapes. The third candidate, Josefina Vazquez Mota, the candidate of Mr Calderon's governing National Action Party, who took just 25 per cent of the vote, has accepted defeat.