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.
Institutional Revolutionary Party (PRI), Mexico’s old ruling party, is set to return to power as early official results indicate its candidate Enrique Pena Nieto has won the presidential election.
Enrique Pena Nieto, 45, is on some 38%, several points ahead of Andres Manuel Lopez Obrador, who has not conceded.
Thousands of police were on duty for the vote, amid fears of intimidation from drug gangs.
Mexicans were also electing a new congress and some state governors.
Celebrations at the headquarters of the PRI started after the polls closed.
Enrique Pena Nieto declared: “We all won in this election. Mexico won.”
“This is just the start of the work we have before us.”
He thanked Mexican voters for giving the PRI a second chance, saying his administration would have a “new way of governing”.
The election campaign was dominated by the economy and the war on drugs.
“There will be no pact nor truce with organized crime,” Enrique Pena Nieto said.
Enrique Pena Nieto is on some 38 percent, several points ahead of Andres Manuel Lopez Obrador
He had been presented as the new face of the PRI, a break with the party’s long and at times murky past that included links with drug gangs.
The party held on to power for 71 years until it was defeated in 2000.
Enrique Pena Nieto built his reputation on the “pledges” he set out for his governorship in Mexico state, focusing on public works and improvement of infrastructure.
Outoing President Felipe Calderon has congratulated Enrique Pena Nieto and promised to work with him during the transition to his inauguration in December.
“I sincerely hope for the smooth running of the next government for the benefit of all Mexicans,” Felipe Calderon said, in a televised address.
Andres Manuel Lopez Obrador, running for the leftist Party of the Democratic Revolution (PRD) is in second place with about 31% of the vote.
The official quick count, published by the electoral authorities (IFE), is based on returns from a sample of around 7,500 polling stations across Mexico.
Andres Manuel Lopez Obrador, who was the runner-up in the 2006 election, has not conceded victory.
“The last word hasn’t been spoken yet,” he said.
“We simply do not have all the facts. We are lacking the legality of the electoral process.”
In 2006, he refused to recognize Felipe Calderon’s victory and led street protests for months afterwards.
Josefina Vazquez Mota, the candidate of the governing National Action Party (PAN) had already accepted defeat.
The initial results from IFE put her on some 26%.
Almost 80 million people were eligible to cast their ballots on Sunday.
Police and army were deployed to protect voters from intimidation by drug cartels at polling booths.
Officials said the voting was largely peaceful, but reported some initial problems as a number of stations opened later than planned.
With nearly half the Mexican population living in poverty, the economy was 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 was 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 were also electing 500 deputies, 128 senators, six state governors, the head of government in the Federal District (which includes Mexico City) and local governments.