* Leftist Lopez Obrador accuses Pena Nieto of buying votes
* Plans to hold more than 140 rallies
* Accusations stalling Pena Nieto's transition
By Lizbeth Diaz
MEXICO CITY, July 20 (Reuters) - Mexico's presidential runner-up said on Friday he would launch a media offensive and stage rallies around Mexico to try to overturn the victory of Enrique Pena Nieto, whom he accuses of buying votes to win the presidency.
Unveiling his plan for a "defense of democracy," leftist Andres Manuel Lopez Obrador said television and radio spots would be used to inform the public of the vote buying and money laundering he said was behind Pena Nieto's win.
Pena Nieto's Institutional Revolutionary Party (PRI) has dismissed Lopez Obrador's allegations, calling the former mayor of Mexico City and 2006 election runner-up a sore loser, and threatening to sue him for defamation.
But the attacks have sullied President-elect Pena Nieto's victory and stalled his transition before he takes office on Dec. 1. Pena Nieto has said he won't name an official transition team until he is officially declared the winner.
Analysts, however, do not expect electoral authorities to change the result.
Lopez Obrador has been unrelenting, claiming that even voters for the PRI, which held Mexico in an iron grip for most of the 20th century until it was ousted in a 2000 election, were "horrified" by how the election played out.
"We can't accept someone who establishes a market republic where money resolves everything, we won't accept living in a country characterized by corruption," he told a news conference.
From the end of this month, Lopez Obrador said his leftist coalition would hold meetings in more than 140 of the main public squares and spaces around the country to inform Mexicans about the methods he accuses his rivals of using to win.
LOTS OF COFFEE
Lopez Obrador lost the July 1 vote by more than 3 million votes but claims Pena Nieto secured 5 million votes by illegal means. In 2006, Lopez Obrador lost the election to President Felipe Calderon by less than 250,000 votes and staged massive street protests that choked the capital for weeks.
Mexico's electoral tribunal has until Sept. 6 to rule on his challenge, and Lopez Obrador has so far refrained from calling for mass demonstrations to support his cause.
However, he has hinted at stepping up his protests, this week saying he would "target the responsible authorities" if the election result was allowed to stand.
Jesus Murillo, Pena Nieto's legal adviser, said Lopez Obrador's claims were going nowhere.
"Given the lack of votes and the lack of evidence, what (they) are trying to do is put on a show to insist on the vote-buying notion, which hasn't been proven anywhere and obviously doesn't exist," he told Milenio television.
Lopez Obrador has delivered boxes of evidence to electoral authorities and cited documents that he says show illicit funding entering Pena Nieto's campaign.
On Thursday, Calderon's conservative National Action Party (PAN) joined the Party of the Democratic Revolution (PRD) in accusing the PRI of using laundered money in the election. PRD was part of a coalition of leftist parties that backed Lopez Obrador.
Lopez Obrador said the net was closing around the PRI, whose rule of Mexico between 1929 and 2000 was marred by frequent allegations of corruption and vote rigging.
"These people who violated the constitution are really nervous, my information is that they're meeting every half hour, they're drinking hundreds of liters of coffee," said Lopez Obrador. "Because they're caught, we got them, they thought it was going to be easy to buy the presidential election." (Additional reporting by Miguel Angel Gutierrez; Writing by Dave Graham; Editing by Eric Beech)