Mexicans cast ballots for nine state governorships and more than 1,000 state and municipal posts. Polls closed at 2300 GMT (1900 ET) and preliminary results are expected around 0300 GMT on Monday.

At least seven candidates and nine campaign officials were murdered in campaigning soured by drug cartel intimidation and dissident teachers protesting against education reforms.

Fresh violence flared over the weekend, with 16 people killed on Saturday when gangs clashed near the resort city of Acapulco, but the incident appeared to be unrelated to the election.

Heavily armed convoys patrolled as 40,000 police and troops fanned out across southern Mexico to safeguard the vote.

Activists stole or set fire to dozens of ballot boxes in the restive states of Guerrero, Oaxaca and Chiapas early on Sunday, but the vote was otherwise largely peaceful, officials said.

After pushing through a raft of economic reforms early in his presidency, Pena Nieto has been battered by allegations of corruption and for failing to bring drug violence under control.

"Security and corruption are the most important things that must be improved," said Luis Castillo, an 80-year-old pensioner, after voting for the conservative opposition National Action Party (PAN) in Mexico City.

"The PRI have spent 70 years being corrupt and that is enough," he added, referring to Pena Nieto's ruling centrist Institutional Revolutionary Party (PRI).

Pena Nieto was buffeted by criticism over the apparent massacre of 43 students in September by a drug gang working with local police. Then he faced accusations of corruption following revelations that he, his wife and his finance minister had bought houses from government contractors.

There were 1,374 murders across Mexico in April, the highest monthly total in nearly a year, police data show.

While Pena Nieto's approval rating has plummeted, polls suggest the PRI may keep the thin majority it musters with its allies in the lower house, partly due to weakness and splits in the opposition.

The PRI, with its partners the Green Party and the smaller New Alliance party (PANAL) won 251 of 500 lower house seats in the 2012 elections with around 42 percent of the total vote. Polls suggest they will be close to that total again.

Having fulfilled the bulk of his main legislative pledges, including measures to end the state oil and gas monopoly and open up the telecoms sector to competition, Pena Nieto is not expected to rely on Congress as much in his last three years.

However, the government still has legislation pending, including bills aimed at encouraging investment in rural areas.

The reforms have yet to kick-start strong economic growth and the government recently cut its forecast for the third year running. Analysts predict growth of little more than 2.5 percent in 2015.