About one-third of the ballots cast in Mexico's general elections last weekend, according to estimates, will have to be recounted for a variety of reasons specified by the law, election officials said Tuesday.

The recount is a normal procedure under election rules, with the ballots cast at one-third of polling places being tallied a second time after the 2009 legislative elections.

Recounts can be executed for a number of reasons, including when there is a difference equal to or less than 1 percent separating the winner and the second-place finisher, when there are errors on ballots or when the number of void ballots is greater than the difference between the victor and the candidate who came in second.

Between 45,000 and 50,000 packages from the nearly 143,000 polling places established for the general elections "would eventually be opened" for the recount of ballots, Federal Electoral Institute, or IFE, official Alfredo Figueroa said in a press conference.

"We are making estimates ... We are talking in general terms about a third of the presidential election," Figueroa said.

As of now, full recounts will be done in 19 of Mexico's 300 election districts because the difference between the winner of the presidential election and the second-place finisher is equal to or less than 1 percent, meaning that the ballots from 10,000 polling places will be tallied a second time.

"There will probably be partial recounts in other districts," Figueroa said.

The recount announced by the IFE is not related to threats by the left, which has not admitted defeat in the election, to challenge the results.

Leftist presidential candidate Andres Manuel Lopez Obrador alleged Monday that the general elections were "plagued by irregularities" and might be challenged if the preliminary results stand.

Institutional Revolutionary Party, or PRI, candidate Enrique Peña Nieto won Mexico's presidency based on the preliminary figures.

Peña Nieto garnered 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 IFE.

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."

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 that contest.

"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.

The decision to recount ballots is made by the election councils in each district, with the IFE simply providing an estimate Tuesday of how many ballots might be included.

This is the first time that recounts can be done in a presidential election, a change brought about by the last election law reforms enacted in Mexico.

Ballots from more than 40,000 polling places were recounted following the 2009 legislative elections, the first vote conducted after implementation of the reforms. EFE