Mexican leftist presidential candidate Andres Manuel Lopez Obrador says he will mount court challenges against the results of the July 1 election, claiming vote-buying and campaign overspending by the winner of official vote counts, Enrique Pena Nieto.

The announcement came Monday amid rising calls to investigate what appears to have been the distribution of thousands of pre-paid gift cards to voters before the election, and allegations by Lopez Obrador's supporters that some state government officials passed funds to Pena Nieto's campaign effort.

Lopez Obrador finished about 6.6 percentage points behind Pena Nieto of the old guard Institutional Revolutionary Party, or PRI.

A man holds a defaced image of president-elect Enrique Pena Nieto as Mexicans unhappy with the presidential election results prepare for a march, in Mexico City, Saturday. (Marco Ugarte/Associated Press)

Even President Felipe Calderon, who suffered his own post-electoral dispute with Lopez Obrador following the 2006 election, called for an investigation into the vote-buying allegations and reforms to prevent such practices in the future.

"The issue of the [gift] cards, the accounts and all, is an issue that I don't know if it will be sufficient to overturn an election with these margins, but it should be resolved," Calderon said.

"We cannot just accept this and say 'a little bit doesn't hurt,'" Calderon told a local radio station. Referring to excessive campaign spending, Calderon said, "every day more evidence is coming out. It's worrisome."

"I think electoral authorities should have a change of heart and punish this," Calderon said. Referring to possible measures against campaign overspending for future elections, he said "I think this is the big reform that Mexico still lacks."

Calderon offers his thoughts

Calderon also suggested for the first time the issue could play a role in Pena Nieto's transition period. Pena Nieto is to take office Dec. 1 after winning office largely on promises that the PRI had left behind the vote-buying and repressive tactics it used to retain Mexico's presidency from 1929-2000.

"I think this could be a smooth transition, if this is resolved correctly," Calderon said of the allegations.

Calderon's conservative National Action Party will file court challenges regarding electoral violations, but won't seek to overturn or annul the vote, said Cecilia Romero, the party's secretary general. Lopez Obrador has said he would seek to have the vote annulled or declared invalid.

National Action quoted party leader Gustavo A. Madero as saying over the weekend that if electoral authorities uphold the results, it would give "legality, but not legitimacy to the presidential election, due to the strong questions about the way millions of votes were obtained."

"We Mexicans were witness to a sophisticated operation of vote-buying, in which Institutional Revolutionary Party governors could be implicated," he said. The party's candidate, who came in third, has already accepted the vote results.

Lopez Obrador told a Monday press conference that "we have evidence to say, and at the right time, to prove, that about five million votes were bought."

In addition to the previous charges that the PRI handed out thousands of pre-paid gift cards to voters in slums on the outskirts of Mexico City, Lopez Obrador's supporters showed a video Monday in which cheques from the northern state of Zacatecas can be seen made out to a prominent local PRI member. The video also shows receipts and spending reports showing expenditures for what appear to be campaign events, rallies and travel expenses.

Asked about the new allegations Monday, PRI party spokesman Eduardo Sanchez said "Lopez Obrador is seeking to justify his defeat by saying five million Mexicans sold their votes."

Vote counts

While final vote counts and recounts were finished Sunday for both the presidential and congressional races, it remains unclear exactly how many legislative seats each party won. Under Mexico's electoral system, some legislators are elected directly and others are assigned seats according to the percentage of total votes won by their party, a calculation based on a complex formula whose results aren't expected for weeks.

Madero said he did not believe the PRI had won a majority in congress, while Calderon said the PRI appeared to be "nearing 50 per cent."

National Action suffered a severe blow in the election campaign, when former president Vicente Fox, a National Action member who ended the PRI's seven-decade hold on the presidency in 2000 elections, issued public calls to support Pena Nieto.

Fox said at the time that Mexicans should "close ranks" around the front-runner for the nation's good.

National Action spokesman Raul Reynoso said the party's 42-member executive committee would probably vote Monday on whether to start expulsion procedures against Fox for what is widely perceived as his betrayal of the party.

Reynoso said the process could take a month or more, depending on whether Fox mounts a defence.