Mexico's top electoral court has begun reviewing complaints of vote-rigging during the July presidential election.
Whoever is declared winner stands to lead a sharply divided country
The Federal Electoral Tribunal's seven judges will rule on all 375 challenges to the official results. It is not known how long the process will last.
There have been mass opposition rallies since the official count gave a narrow lead to conservative Felipe Calderon.
Leftist candidate Andres Manuel Lopez Obrador has led the protests demanding a recount of all 41m ballots cast.
The electoral court must formally declare the winner by 6 September.
Mr Lopez Obrador's campaign had filed complaints at around 50,000 polling stations but the court ordered a recount at just 11,839 of them - about 9% of the national total.
Mr Calderon has already said he won fairly.
Mr Lopez Obrador's supporters have erected a tent city
However, Mr Lopez Obrador says the ballot was rigged and that the country's political system is rotten.
Thousands of his supporters are still camped out on Mexico City's historic main square, the Zocalo, hoping to change the result.
At a rally on Sunday, he told them that he had categorically won the election and that there was a danger of "the people's rights" being "trampled".
He also suggested that the left might set up a civil resistance movement or parallel government.
Mexico's political crisis may not be ended with the results of the partial recount especially if it gives ammunition to both candidates, the BBC's Duncan Kennedy reports.
That would then take the country's still young multi-party democracy into new territory and keep the attention of the Americans and other neighbours firmly fixed on what happens next, our correspondent adds.