Enrique Pena Nieto, the candidate of the Institutional Revolutionary Party, has claimed victory in Mexico's presidential elections.
With around 38% support, he had a lead of least 6% over his nearest rival.
The election completed a dramatic comeback for the Institutional Revolutionary Party (PRI), which ran Mexico as a virtual one-party state for 71 years before it was finally ousted in a 2000 election.
"Mexicans have given our party another chance. We are going to honour it with results," Mr Pena Nieto told supporters in the capital shortly after the quick count was announced.
"The result is irreversible," said Benito Nacif, one of the Federal Electoral Institute's board members.
Departing President Felipe Calderon of the National Action Party (PAN) congratulated Mr Pena Nieto on his victory, but the second-placed challenger Andres Manuel Lopez Obrador refused to concede.
"The last word has not yet been spoken," he told supporters.
Mr Lopez Obrador could choose to challenge the election, as he did six years ago when he narrowly lost to Mr Calderon and launched months of protests against alleged fraud.
He has said in recent weeks that this election campaign was plagued with irregularities, raising concerns that he might again call his supporters onto the streets.
Josefina Vazquez Mota of the PAN trailed in third place in the election with no more than 26% of the vote, the official quick count said.
Her campaign suffered from the often barbaric violence in Mexico's drug war, which has claimed around 55,000 lives since Mr Calderon deployed the army against trafficking cartels early in his presidency.
Mr Pena Nieto persuaded many Mexicans his government would do a better job of reducing the violence.
"The fight against crime will continue, yes, with a new strategy to reduce violence and above all protect the lives of all Mexicans," Mr Pena Nieto said in his victory speech.
He also dismissed critics' suggestions that the PRI might try to reach a deal with one or more drug cartels in an effort to eliminate more troublesome rivals. "Let it be very clear: There will be no deal, no truce with organised crime."
Mr Pena Nieto has also promised to boost economic growth, open state-owned oil monopoly Pemex to foreign investors, raise tax revenue and liberalise the labour market.
