A little girl in a wheelchair presented Francis with a handmade Valentine's Day card.
Francis urged the hundreds of thousands of people who gathered in the gritty suburb of Ecatepec to be on the "front lines" in forging Mexico's future.
In his homily, Francis warned the faithful that the devil, "the father of lies," tries to divide society and that Mexicans must resist the temptations of wealth, vanity and pride.
The Mexico City government says via Twitter that there was a grassland fire in San Salvador Atenco about 6 miles (10 kilometers) from the site of the papal Mass.
The pope walked out of the nuncio's residence to bless and kiss people waiting there: families with children, people in wheelchairs.
The Mass is to take place in an outdoor field with an estimated capacity of 400,000.
"We could go until 4 a.m.?" the pope asked. "Yesss," the crowd answered. "Well, but that could be a little long," he responded.
The Pope is conducting a mass in the Basilica of Guadalupe in front of 30,000 faithful.
He challenged church leaders to not hide behind their own privilege and careers.
Francis moves around the Vatican in a Ford Focus and tends to stick with economy cars when traveling.