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.
Six years ago, after losing the presidential election by a narrow margin, the left-wing Mr Lopez Obrador led weeks of protests that caused disruption in central areas of Mexico City.
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 right-wing National Action Party, has admitted defeat.
But the BBC's Will Grant in Mexico City says there is a broad spread of people, not necessarily from the left, who feel that votes in their parts of Mexico were tampered with.
But Mr Lopez Obrador, of the Party of the Democratic Revolution, said the election had been fraudulent and that he would file an appeal next week.
He accuses the party of Mr Pena Nieto, the Institutional Revolutionary Party, of paying for votes using gift cards for a supermarket chain.
Numerous videos have emerged of people claiming they received credit in exchange for voting for the PRI.
The party governed Mexico for 71 years until it was defeated in the 2000 presidential poll.