Mexico's conservative presidential candidate, Felipe Calderon, snatched a razor-thin election victory today, but his leftist rival vowed to fight the result in the courts and on the streets.
A Harvard-educated conservative, Mr Calderon had 35.83 per cent support with results in from 99.6 per cent of polling stations.
Although he was less than 0.5 points ahead of Lopez Obrador, his lead was insurmountable.
A relaxed Mr Calderon led supporters in a noisy party at the ruling National Action Party offices, and immediately called on his adversaries to forget an ugly and fiercely contested election that has plunged Mexico into a political crisis.
"If the contest is behind us, our differences are behind us. Now is the hour for unity and agreements between Mexicans," said Mr Calderon, a pro-US former energy minister.
Leftist former Mexico City Mayor Andres Manuel Lopez Obrador angrily insisted he won the election, claimed it was plagued with irregularities and pledged to fight the result in Mexico's electoral tribunal.
He called a rally of supporters in Mexico City's vast central square on Saturday, raising fears of street protests and further unrest as well as weeks of legal wrangling like that which followed the US election in 2000.
"We cannot recognize or accept these results," he said.
Mr Calderon (43) would be an ally of the United States in Latin America, where leftists have taken power in Argentina, Bolivia, Brazil, Uruguay and Venezuela and turned away from Washington.
At home, Mr Calderon promises to clamp down on violent criminals and powerful drug trafficking gangs as well as create millions of jobs with pro-business reforms, more foreign investment and a boom in construction.