Thousands are marching through Mexico City's center and protesting what they call the imposition of the candidate of Mexico's old ruling party as the new president.
Marchers are carrying signs and chanting that presumed victor Enrique Pena Nieto won the July 1 election through fraud and vote-buying.
He was the candidate of the Institutional Revolutionary Party, or PRI.
Rival leftist candidate Andres Manuel Lopez Obrador finished more than six points behind.
The PRI has vehemently denied the charges and on Friday accused Lopez Obrador of trying to disqualify the entire electoral process with lies.
Authorities did not release an official crowd estimate, but the march appeared to draw fewer people than similar protests with as many as 90,000 participants.
The events have attracted people from a new student movement and leftist groups supporting Lopez Obrador.