Tens of thousands of people in Mexico City are demonstrating against the result of the presidential election, which was won by Enrique Pena Nieto.
The demonstrators, who are not necessarily linked to any particular party, say the vote was not fair.
They accuse Mr Pena Nieto's party, the PRI, of buying votes; some carried banners saying "Not another fraud".
Mr Pena Nieto, who was declared the winner after a recount of nearly half the votes, denies the allegation.
The second-placed candidate in Mexico's presidential election, Andres Manuel Lopez Obrador, has said he will mount a legal challenge to the result.
He said he would prove that illicit money was used to buy votes in the 1 July poll and secure the victory of centrist candidate Enrique Pena Nieto, who denies this.
Mr Lopez Obrador previously led weeks of protests that caused disruption in central areas of Mexico City, after losing the presidential election by a narrow margin six years ago.
The BBC reports that Mr Pena Nieto was confirmed the winner on Friday after a final recount, with 38.21% to Mr Lopez Obrador's 31.59%.
Third-placed Josefina Vazquez Mota from the National Action Party, has admitted defeat.
The PRI has been out of power for 12 years. The party previously ruled Mexico for seven decades.
Mr Pena Nieto is due to take power in December, replacing Felipe Calderon of the National Action Party.
The constitution barred Mr Calderon from running for a second term.