Enrique Pena Nieto was finally declared Mexico's president-elect on Friday after a two-month legal fight, but his leftist rival refused to accept the result and called for a rally.

The federal electoral tribunal officially named Pena Nieto the winner of the July 1 election after dismissing a bid by leftist leader Andres Manuel Lopez Obrador to scrap the result over vote-buying claims.

The decision clears the way for Pena Nieto to begin his six-year term on December 1, marking the return of the Institutional Revolutionary Party (PRI) to the nation's highest office after a 12-year absence.

Lopez Obrador claims that the PRI, which governed Mexico with an authoritarian grip from 1929 to 2000, bought five million votes and violated campaign spending rules in order to secure Pena Nieto's victory.

"The elections were neither clean nor free nor genuine, therefore I will not recognize an illegitimate administration that emerged from votes that were bought and other grave violations of the constitution," he told reporters.

The former Mexico City mayor called on his followers to gather for a demonstration in the capital's historic main square, the Zocalo, on September 9, and to decide the way forward.

"Civil disobedience is an honorable duty when it is aimed against thieves who steal the hope and happiness of the people," he said. "I call on all supporters of democracy to gather at the Zocalo."

Lopez Obrador led mass protests that paralyzed Mexico City in 2006 after he lost that year's election to Felipe Calderon, of the conservative National Action Party, by a mere 0.06 percentage points.

But he was unable to change the outcome.

This time, the electoral court confirmed that Pena Nieto, who rejected is opponent's allegations, defeated Lopez Obrador by 3.3 million votes. Pena Nieto won 38.2 percent of the vote compared to 31.6 percent for Lopez Obrador.

"This electoral process was not free of problems, but they were adequately resolved within the law," said judge Maria del Carmen Alanis Figueroa.

Judge Flavio Galvan Rivera said: "It is my personal conviction that the election was free, fair and genuine."

Pena Nieto was scheduled to arrive at the court later Friday to receive his credentials. A few hundred protesters gathered outside the tribunal amid a heavy police presence.

Pena Nieto will inherit from Calderon a brutal drug war that has left more than 50,000 people dead since 2006. Calderon did not run due to term limits.

The PRI's 71-year rule was marred by allegations of vote-rigging and rampant corruption, but Pena Nieto has promised to break with his party's checkered past during his six-year term.

"It is time to begin a new phase of work in favor of Mexico," Pena Nieto wrote on Twitter late Thursday after the court scrapped Lopez Obrador's challenge, offering dialogue 'for the unity and greatness of Mexico'.

Lopez Obrador's coalition claimed that the PRI distributed gift cards to voters in return for their votes. The left also charged that children were sent to polling stations to check how people voted.

But the seven-judge electoral court ruled that the leftist coalition failed to prove its allegations.

Outside the court Thursday night, some 300 protesters shouted 'Mexico without the PRI', brought down barriers and threw water bottles, corn cobs and stones toward the court, as riot police watched passively.

The movement #Yosoy132, which organized protests against Pena Nieto during this year's campaign, has warned that it will lead a 'funeral march to bury democracy'.