Tens of thousands of people in Mexico City are marching 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 Enrique Pena Nieto’s party, the PRI, of buying votes; some carried banners saying “Not another fraud”.
Enrique 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.
Tens of thousands of people in Mexico City are marching against the result of the presidential election, which was won by Enrique Pena Nieto
Andres Manuel Lopez Obrador 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 Andres Manuel Lopez Obrador led weeks of protests that caused disruption in central areas of Mexico City.
Enrique Pena Nieto was confirmed the winner on Friday after a final recount, with 38.21% to Andres Manuel Lopez Obrador’s 31.59%.
Third-placed Josefina Vazquez Mota, from the right-wing National Action Party, has admitted defeat.
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 Andres Manuel 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 Enrique 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.
Mexico has opened the polls in a presidential election dominated by the economy and war on drugs.
Ex-governor Enrique Pena Nieto – seen as the frontrunner – is attempting to win the presidency back for the PRI party that ruled for decades.
His main opponents are left-wing politician Andres Manuel Lopez Obrador, and Josefina Vazquez Mota from the ruling conservative PAN party.
Voters are also choosing a new congress and some state governors.
Nearly 80 million voters are eligible to cast their ballots.
Enrique Pena Nieto, the 45-year-old former governor of Mexico state, is seeking to bring back the presidency to the PRI (Institutional Revolutionary Party), which held on to power for 71 years until defeat in 2000.
“My priority will be to battle the poverty in our country at its roots,” he said during his final campaign rally.
But in recent weeks the gap between Enrique Pena Nieto and Andreas Manuel Lopez Obrador, a 58-year-old former Mexico City mayor, has been narrowing, opinion polls say.
Mexico has opened the polls in a presidential election dominated by the economy and war on drugs
Andreas Manuel Lopez Obrador, of the Party of the Democratic Revolution (PRD) has seen his campaign boosted by a student-led movement against the PRI.
The PRD candidate came close to winning the poll six years ago. He accused the governing party of fraud and vote-buying and led a month of street protests against the official result.
But he said things have changed: “In 2006 we lacked organization, now we are organized. 2012 is not 2006.”
The candidate of the ruling National Action Party (PAN) is 51-year-old Josefina Vazquez Mota, a businesswoman who has promised to tackle corruption.
With nearly one third of the Mexican population living in poverty, the economy has been one of the main issues in the campaign.
Unemployment remains low at roughly 4.5%, but a huge divide remains between the rich and the poor.
Another issue dominating the campaign is the war on drugs launched nearly six years ago by President Felipe Calderon, who is constitutionally barred from seeking re-election.
The main opposition candidates have been critical of Felipe Calderon’s policies.
They point out that more than 55,000 people have been killed in drug-related violence since 2006.
Mexicans are also electing 500 deputies, 128 senators, six state governors, the head of government in the Federal District and local governments.