Left-winger Andrés Manuel López Obrador has a clear lead in the race to be Mexico's next president, exit polls say.
The former mayor of Mexico City, who was the front-runner ahead of the vote, is estimated to have polled at least 53%, one survey by Parametria said.
All exit polls put him well ahead of his nearest rivals.
Ruling party candidate José Antonio Meade said he accepted Mr López Obrador had won and wished him well.
The first official results are expected at midnight (04:00 GMT).
Sunday's election followed one of Mexico's deadliest election campaigns in decades.
More than 130 political candidates and party workers have been killed.
What is at stake?
Mr López Obrador, 64, has been runner-up in Mexico's two previous elections. He now seems likely to end the dominance of 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".
Many Mexicans have grown disillusioned with incumbent President Enrique Peña Nieto's PRI administration, particularly over the sluggish economy and endemic corruption.
Mr López Obrador put tackling corruption at the centre of his election campaign, promising to improve wages and pensions by stamping out rampant abuse.
His opponents have tried to paint him as a populist who cannot be trusted with the economy.