Thousands marched through Mexico City's centre to protest what they called the "imposition" of the candidate of the old ruling party as the country's new president.
Protesters carried signs accusing presumed President-elect Enrique Pena Nieto of electoral fraud and Mexico television giant Televisa of being a "factory of lies."
Opponents say Pena Nieto's party, the Institutional Revolutionary Party, won the July 1 election through vote-buying and overspending, including paying major media outlets such as Televisa for favourable coverage.
"Mexico didn't vote for fraud. Mexico wants a country that is honest and democratic," said marcher Marlem Munoz, 26, who studies dentistry at Mexico's National Autonomous University. "What happened in the elections was a total mockery directed at the Mexican people."
The PRI has vehemently denied the charges and on Friday accused losing leftist candidate Andres Manuel Lopez Obrador of trying to "disqualify the entire electoral process with lies." Televisa also has denied charges of being paid for positive coverage.
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Mexico City authorities did not immediately release an official crowd estimate, but the march appeared to draw far fewer people than similar protests before the election with as many as 90,000 participants.
A July 7 march, the first after election, drew 50,000. The events have attracted people from a new student movement, "I Am 132," and leftist groups supporting Lopez Obrador.
The lower participation raises questions of whether Mexico's university students have spawned a real movement in their demand for "authentic democracy" and an opening of Mexico's media, or if it's just part of the standard post-election protests in Mexico.
In 2006, after Lopez Obrador narrowly lost to President Felipe Calderon, he marshaled hundreds of thousands of supporters to block Mexico City's main center for weeks. Lopez Obrador said he will not mobilize people to the streets this time.
His choking of central Mexico City in 2006 was highly unpopular with everyday residents.
I Am 132 has released a series of proposed events over the coming weeks, including yesterday's march, designed to overturn the vote results.
Other groups have said they will block the Dec. 1 inauguration of Pena Nieto.
Pena Nieto, 46, won the presidential election by 6.6% points, according to the official count, bringing the PRI back to power after 12 years in opposition.
The party had ruled Mexico for 71 consecutive years, with what critics say was the help of corruption, patronage and vote fraud.
The final vote count must be certified in September by the Federal Electoral Tribunal.