Mexico's general elections were "plagued by irregularities" and may be challenged if the preliminary results stand, leftist presidential candidate Andres Manuel Lopez Obrador said.

Institutional Revolutionary Party, or PRI, candidate Enrique Peña Nieto won Mexico's presidency with 38.14 percent of the vote, while Lopez Obrador took second place with 31.64 percent, according to the final preliminary results released Monday by the Federal Electoral Institute, or IFE.

Some 98.95 percent of the ballots had been counted as of the 8:00 p.m. cut-off time for the Preliminary Election Results Program, or PREP, the IFE said.

Lopez Obrador, a former Mexico City mayor who was the standard-bearer of a leftist coalition led by the Party of the Democratic Revolution, or PRD, said in a press conference Monday that he would not accept "fraudulent results."

"We have to have the grounds, we would never make a complaint if we did not have evidence," Lopez Obrador said in response to a question about whether he thought the election may have been tainted by fraud.

Lopez Obrador lost the 2006 presidential election to Felipe Calderon, of the National Action Party, or PAN, by 0.56 percent and has never recognized the results, claiming victory in the contest.

PREP figures are informative in nature and not considered final results, and they are not legally valid or a substitute for the tallies from Mexico's 300 election districts, which will begin their counts on Wednesday.

Parties and candidates can request recounts from specific districts once the official tally is released.

Lopez Obrador said in Monday's press conference that he would focus on the final official results and not on the PREP.

"Yes, yes, we are going to challenge them," Lopez Obrador said in response to a question about whether he would challenge the results if the final numbers were similar to the preliminary figures.

The veteran politician, however, said he would pursue all legal options before challenging the results.

"As you can understand, I cannot accept any results unless I am certain that the votes of the citizens have been respected ... We are going to follow the entire process established by the law," Lopez Obrador said.

The leftist politician alleged that the PRI bought votes in the election.

"The PRI candidate used pots of money, billions of pesos, of an illicit origin, exceeding by a lot what the law allows," Lopez Obrador said.

"If they clean up the votes that were bought, we won by a wide margin. We cannot accept fraudulent results," Lopez Obrador said.

The PRI, which governed Mexico from 1929 to 2000, lost the 2000 and 2006 presidential elections to the conservative PAN.

"Let's not get ahead of ourselves, let's wait," Lopez Obrador said in response to a question about whether he was open to accepting the IFE numbers and rulings of the electoral courts.

Peña Nieto will succeed Calderon on Dec. 1. EFE