Throngs lined the concrete-laden streets of Ecatepec, whose walls were decorated with graffiti art featuring the pope's image, to cheer the pontiff on the second full day of a trip that will take him to other Mexican hotspots.
The rough Mexico City suburb of 1.6 million people has become infamous for a spate of disappearances of women, whose bodies have turned up in abandoned lots or canals.
The city lies in the populous state of Mexico, where some 600 women have been killed between January 2014 and September 2015, according to the non-governmental National Citizen Observatory of Femicides.
Thousands of pilgrims spent the night outdoors, wrapping themselves in blankets and using cardboard as makeshift tents against freezing temperatures on the field where the mass will be held.
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Many said that despite the city's bad reputation, they were not concerned about sleeping outside. Hundreds of police officers stood guard around the field.
"We know that Ecatepec has a lot of problems like the lack of security and kidnappings," said Rodrigo Perez, a 25-year-old public security student.
The Argentine-born pontiff made it clear before his arrival in Mexico that he would speak out about the corruption and crime afflicting parts of the country.
He used his visit to the National Palace and the capital's cathedral on yesterday to bluntly tell political and religious leaders to provide Mexicans with "true justice" and combat drug violence with "prophetic courage."
Many Mexicans, fed up with a decade of drug violence that has left 100,000 dead or missing, had hoped to hear such words from the 79-year-old pontiff.
The crimes against women in the state of Mexico, which surrounds the capital, prompted the federal government to declare a "gender violence alert" requiring protective measures in 11 towns, including Ecatepec.