The former mayor of Mexico City has registered his candidacy for next year's presidential race.
There was an unprecedented mass rally in support of the mayor
Andres Manuel Lopez Obrador signed up as a candidate for the Democratic Revolution Party's nomination, after stepping down as mayor on Friday.
Opinion polls suggest he is currently the favourite to win the poll.
Earlier this year he was the subject of a failed legal attempt to bar him from standing. About one million Mexicans rallied in the capital to support him.
"We know what needs to be done to make things better in this country," he said on Saturday.
On Friday Mr Lopez Obrador addressed thousands of his supporters in a speech to mark the end of his time as mayor.
He listed his achievements: more schools and transport infrastructure for the city; more help for single mothers; and pensions for the elderly.
He said he was most proud of having put the poor first.
With a clear eye on next year's poll, he went out of his way to point out that he knows how to be fiscally prudent, the BBC's Claire Marshall reports from Mexico City.
Mr Lopez Obrador went on the defensive over crime. With kidnappings now an everyday occurrence, many feel the capital to be more unsafe than ever, our correspondent says.
He dismissed this concern. He claimed that crime figures were at their lowest for 10 years - and that some people were trying to manipulate these statistics.