Mexico's electoral tribunal on Thursday unanimously rejected a legal bid to overturn Enrique Pena Nieto's victory in the July 1 presidential election, paving the way for the centrist to take office and press ahead with his reform agenda.

Pena Nieto, 46, and his Institutional Revolutionary Party, or PRI, were accused by runner-up Andres Manuel Lopez Obrador of laundering money and buying votes to secure victory, but the judges said there was insufficient evidence of wrongdoing.

Lopez Obrador, a leftist former mayor of Mexico City, accused the 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 court must settle some smaller issues before Pena Nieto is officially declared president-elect and can take office in December, but the remaining issues are largely formalities.

"There is no proven vote buying, no evident coercion or illicit inducement," Justice Flavio Galvan said, rejecting the challenge.

Hundreds of protesters shouted their disapproval outside the tribunal's offices when the decision was announced.

Pena Nieto won the election by some 3.3 million votes, or about 6.5 percentage points. He rejected the claims of Lopez Obrador, who also unsuccessfully challenged the 2006 election result, which he lost by less than 1 percentage point.

Election experts say allegations of vote buying are nearly impossible to prove unless someone is caught in the act.

The youthful-looking Pena Nieto has had to hold back on his plans to forge deals in Congress over economic reforms due to the defiance of Lopez Obrador, who drew thousands of protesters to the streets after the July 1 election.

The protests tapped into memories of the PRI's long rule in Mexico, which lasted between 1929 and 2000 and was frequently dogged by allegations of corruption and vote-rigging.

In 2000, officials were accused of funneling more than $100 million from state oil firm Pemex to fund the PRI's unsuccessful presidential bid, when it lost to the conservative National Action Party (PAN).

Electoral authorities fined the PRI, but prosecutors failed to convict officials linked to the crime.

Lopez Obrador's allies have been accused of using similar tactics. In 2004, a close aide of Lopez Obrador was caught on camera stuffing wads of cash into a suitcase.

Once almost omnipotent in Mexico, the PRI has spent the last 12 years in opposition. Although Pena Nieto led it back to power in the July election, the party fell short of a majority in the new Congress, so it will rely on support from other parties to pass legislation.

The new Congress convenes on Saturday.

Pena Nieto has pledged to push through reforms to spur growth, such as extending the country's tax base, opening up state oil giant Pemex to more private investment, and liberalizing antiquated labor laws.

His party has also said it will seek to bring more transparency and accountability into politics in Congress.