Voting has begun in nearly half of Mexico's states to elect governors and mayors, amid high levels of security.
The elections, in 14 of 31 states, have been overshadowed by violence and intimidation, with two candidates killed during the campaign.
Correspondents say the campaign was the bloodiest in more than 15 years.
The vote is seen as an unofficial referendum on President Felipe Calderon's security crackdown on drug-related violence.
Some 5,000 deaths so far this year have been blamed on drug-related violence, putting 2010 on course to be the deadliest in half a decade.
'Nervous'This year the killings have touched the electoral process itself.
Rodolfo Torre, a front-runner candidate for governor in the north-eastern state of Tamaulipas, was killed after gunmen ambushed his convoy on 28 June. Four of his aides also died.
His brother, Egidio Torre, is replacing him as candidate.
The BBC's Julian Miglierini in Mexico city says the killing was the highest-profile political killing since the assassination of presidential candidate Luis Donaldo Colosio in 1994, and has deepened concerns over how Mexican democracy is suffering at the hands of organised crime.
Our correspondent says that turnout figures could show how that fear is present in Mexican voters' minds.
A mayoral candidate in the same state was also killed, while others stepped down and 550 electoral officials have resigned, according to the daily newspaper La Reforma daily.
"We're a bit nervous, but we have to go out to vote because it's our only weapon for the future," Pedro Esparza, a factory worker in Nuevo Laredo, a town close to the border, told AFP news agency.
Some politicians have told the BBC they had to keep their campaign diaries secret to avoid possible assassination attempts.
The main rivals to Mr Calderon's National Action Party (PAN) are the Institutional Revolutionary Party (PRI), which held power for decades before Mr Calderon took office in 2006.
After coming to power, Mr Calderon declared a "war on drugs", deploying thousands of troops to violence hotspots.
