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.
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