Jubilation for Mexico's next president.

Leading by six percentage points in early results, Enrique Pena Nieto promised to fight against organized crime and asked voters to leave election tensions behind.

(SOUNDBITE) (Spanish) PRESIDENTIAL CANDIDATE FOR INSTITUTIONAL REVOLUTIONARY PARTY (PRI), ENRIQUE PENA NIETO, SAYING:

"Today democracy won. In this election we all won, Mexico won. For those that voted for other options be sure that I will govern with all and for all."

His win marks a return to power for the Institutional Revolutionary Party after 12 years of opposition rule.

But leftist candidate Andres Manuel Lopez Obrador refused to concede, saying he is waiting for final results.

(SOUNDBITE) (Spanish) PRD CANDIDATE ANDRES MANUEL LOPEZ OBRADOR, SAYING:

"The information that we have indicates something else than what is being said officially. I do not disqualify what they are making known officially but simply we do not have all the facts."

In the 2006 election, Lopez Obrador alleged fraud in his narrow loss to President Felipe Calderon, prompting massive protests.

Crying foul play, hundreds of his supporters again gathered in Mexico City on Sunday.

Lindsey Parietti, Reuters