Polls are due to open across Mexico for Sunday's presidential, parliamentary and local elections.
Front-runner for president is Andres Manuel Lopez Obrador, the left-wing former mayor of Mexico City who has pledged to crack down on corruption.
If he wins he will oust the two parties that have governed Mexico for nearly a century.
However, campaigning has been marred by some of the worst political violence in the country for decades.
BBC Mexico correspondent Will Grant says many voters are keen to replace the government of incumbent President Enrique Peña Nieto. They are angry at Mexico's sluggish economy as well as widespread corruption and crime, he adds.
Andres Manuel Lopez Obrador - often referred to by his initials Amlo - has made tackling corruption the central plank of his campaign, promising to improve wages and pensions by stamping out rampant abuse by the state and by political and business elites.
Mr Lopez Obrador, 64, was runner-up in the last two elections and if he wins this time he will end the dominance of the two parties that have governed Mexico for decades - the Institutional Revolutionary Party (PRI) and the National Action Party (PAN).
He has referred to them as being part of the same "mafia of power", putting forward a left-wing coalition led by his party, Morena, as a chance to make a radical break with the past.
His closest rival looks to be Ricardo Anaya of PAN who heads a centre-right coalition. He has tried to paint Mr Lopez Obrador as a populist and a maverick who cannot be trusted to run the economy.Image copyright Reuters Image caption Andres Manuel Lopez Obrador, (L) has been the front-runner ahead of Jose Antonio Meade (C) and Ricardo Anaya
The candidate for the governing PRI party is José Antonio Meade, a former finance minister.
As well as a new president, Mexican will be voting for 128 senators and 500 deputies in Congress as well as state and local officials. In all, 88 million people will be eligible to vote.
Mexico is the second largest economy in Latin America and a major oil exporter. However, oil prices have dropped and the Mexican currency, the peso, has fallen sharply against the dollar.
More than 40% of the population lives in poverty and high levels of corruption and violence have led some companies to pull out of the worst affected areas.
The run-up to Sunday's elections has seen some of the worst violence in living memory, our correspondent says, as criminal groups attempt to control local politics.
More than 130 candidates and political workers have been killed across the country since campaigning began in September.
On Saturday, a journalist was shot dead in a bar in the village of Saban in the southern state of Quintana Roo, officials said. Mexico has a reputation for being one of the most dangerous countries in the world for journalists.