The leftist runner-up in Mexico's presidential race refused to accept his election defeat on Friday and held out the possibility of further protests after a court threw out his challenge to the result.

Andres Manuel Lopez Obrador accused Enrique Pena Nieto of laundering money and buying votes to win the July election but Mexico's electoral court rejected his challenge to the result late on Thursday, paving the way for Pena Nieto to take office later this year.

"I cannot accept the tribunal's ruling, which has declared the presidential election valid," Lopez Obrador told reporters, calling for a rally in Mexico City's main square on September 9.

"Then we will determine what happens next."

Lopez Obrador did not specify the steps he was considering but protests he organized after narrowly losing a first bid for the presidency in 2006 blocked some of Mexico City's main avenues for several weeks and caused major disruptions.

"Civil disobedience is an honorable duty when directed against the thieves of the hope and happiness of the people," he said.

Lopez Obrador, a leftist former mayor of Mexico City, accused the Pena Nieto and his Institutional Revolutionary Party (PRI) of buying 5 million votes with illegal funding and plying voters with presents ranging from supermarket gift cards to fertilizer, cement and livestock.

The delay in endorsing the July 1 election result has meant Pena Nieto has had to hold back on his plans to forge deals in Congress over economic reforms which are seen as vital to boost growth in Latin America's No. 2 economy.