Mexicans started voting Sunday in presidential and congressional elections, with Andres Manuel Lopez Obrador expected to become the country's first leftist president in decades.
About 89 million people are eligible to vote in what are Mexico's biggest ever elections.
Among the 3,400 offices being contested are 500 seats in the Chamber of Deputies, 128 in the Senate, eight governorships and 1,600 mayoralties as well as other local offices.
Opinion polls have given an almost certain victory to left-wing populist Lopez Obrador, 64, a former Mexico City mayor and third-time presidential candidate, who has had a wide lead over his two centrist rivals.
Former Chamber of Deputies president Ricardo Anaya, 39, is seeking to appeal to young voters - who make up 40 per cent of the electorate - by advocating "modern change" based on technological innovation.
Politically independent five-time minister Jose Antonio Meade, 49, has attempted to keep his distance from outgoing President Enrique Pena Nieto's Institutional Revolutionary Party (PRI), which picked him as its candidate in a bid to improve its corrupt image.
Pena Nieto, whose approval ratings stand at only about 20 per cent, is limited by the constitution to a single term.
A fourth candidate, independent Jaime Rodriguez, is not expected to get more than a few per cent of the vote in the single-round election.
The electoral campaign has been overshadowed by the killings of about 120 politicians since September, according to the risk consultancy Etellekt.
Among the dead were more than 40 candidates or would-be candidates. Criminal gangs are seeking to gain influence on the country's politics especially at a local level, Etellekt director Ruben Salazar said.
The elections take place in an atmosphere of weariness and anger over Mexico's soaring crime rate - with 29,000 killings recorded last year - corruption and sluggish economic growth of about 2 per cent.
"The state of mind is based on being fed up, despair and anger," said election expert Irma Mendez from the Latin American Faculty of Social Sciences in Mexico.
That also concerns young people, who usually take little interest in politics, but at least 70 per cent of whom could vote this time, according to a poll by the platform Nacion 321.
"Of course we want to change the direction our country has taken," said Paola Flores, 24, one of the 40 million Mexicans aged between 19 and 36, some of whom may be voting for the first time.
Lopez Obrador's eventual victory is expected by many to further worsen Mexico's already frosty relations with the United States, whose president, Donald Trump, has slammed the country over illegal migration and pressured it to pay for a border wall.
Lopez Obrador would seek to reduce Mexico's dependency on US fuel and other kinds of trade, his advisor Alfonso Romo said.
The candidate has also proposed a new strategy on fighting crime, including an amnesty for low-level gang members and social programmes.
Polls opened at 8am (1300 GMT) and close at different times across the country, with the first shutting at 6pm. Official results from the presidential elections are expected early Monday.
