Mexico's electoral body has rejected a request by left-wing candidate Andres Manuel Lopez Obrador for a full recount of votes from July's disputed election.
Mr Lopez Obrador's supporters have been staging huge protest rallies
Instead, the electoral tribunal's seven judges ordered a partial recount.
Mr Lopez Obrador rejected the ruling and urged his supporters to continue their protests to demand a ballot-by-ballot recount.
The 2 July vote gave victory to the conservative candidate, Felipe Calderon, by less than 1%.
The electoral tribunal ordered the recount of votes at 11,839 of the country's almost 130,500 polling stations.
After the ruling, Mr Lopez Obrador addressed a rally in central Mexico City - where hundreds of thousands of supporters have been holding protests in recent days - and renewed his call for a full recount.
"We maintain our demand," he said. "We disagree with the tribunal's decision. They must reverse it."
Mr Lopez Obrador said "peaceful civic resistance" would continue and urged people to take part in a further protest on Sunday.
Supporters chanted "Vote-by-vote!" and blocked the entrance to the tribunal.
"If there is no solution, there'll be revolution," they shouted.
Mr Lopez Obrador has challenged the election result, saying the vote was rigged.
Mr Calderon says his victory was irreversible, and his conservative National Action Party has described Mr Lopez Obrador's claims as "schizophrenic".
The dispute has paralysed Mexican politics, correspondents say.
A president-elect must be declared by 6 September to replace Vicente Fox on 1 December.