Pope Francis will celebrate an unprecedented mass straddling both sides of the Mexico-US border on Wednesday to address the plight of migrants fleeing misery and crime in their homelands.
The 79-year-old pontiff wraps up an intense, five-day trip to Mexico with a visit to Ciudad Juarez, the world's former murder capital, where he will pray with inmates at a prison and lead the service at the border.
After pressing Mexican politicians, religious leaders and society to fight for a nation without drug cartels, Francis will turn his attention to migrants, an issue dear to him, on his last day.
A huge stage was set near the border fence for a mass with more than 200,000 people.
Francis is expected to salute people watching on the other side of the Rio Grande in El Paso, Texas. The service will be also broadcast on a giant screen at a 51,000-capacity stadium in the US city.
The pope's decision to make a plea for migrants across from the United States coincides with the US presidential election primaries, in which illegal immigration is a hot button issue.
Republican White House hopeful Donald Trump, who has promised to make Mexico pay to build a wall along the border, called the pope a "very political person."
"I think that he doesn't understand the problems our country has. I don't think he understands the danger of the open border that we have with Mexico," the billionaire real estate tycoon told Fox Business channel last week.
- 'We don't harm anybody' -
While the pope has defended migrants fleeing wars, poverty and religious persecution across the world, he has also called on governments to remedy the root causes of the exodus and for migrants to respect the laws of host countries.
Central Americans have been leaving their poor and gang-infested countries in droves, crossing Mexico's southern border with Guatemala to embark on their journey north.
The trek is filled with dangers from gangs that steal, kill or seek to forcibly recruit them, as well as corrupt officials who demand bribes to let them travel.
Many have also died under the scorching sun while crossing the desert into the United States.
"We don't go there to harm anybody. We leave our countries to seek a better future. We only ask for respect," said Isaias Franco, a 47-year-old Salvadoran having lunch Tuesday near the site of the mass.
Franco lived in the US state of Oklahoma for seven years until he was deported in 2011. Like many migrants, he returned to the northern border after a long journey on top of a freight train known as "The Beast."
But the Mexican government has also cracked down on illegal migration at its southern border.
- Prayer for victims -
During mass, Francis will also pray for crime victims in Mexico, where more than 100,000 people have died or gone missing in a decade of drug violence.
Relatives of 43 students who disappeared in the southern state of Guerrero in 2014 are expected to attend the mass, as well as some of the mothers of the hundreds of women killed in Juarez in the past two decades.
Ciudad Juarez stands as a grim symbol of Mexico's violence, but also of hope.
Billboards were put up across the city to welcome the pope with the words "Juarez is love."
But those signs contrast with black crosses with pink backgrounds in protest at the murders of women that have plagued the city.
The pope's visit to the prison, which was once considered among the most dangerous in Latin America, will have its own symbolism as it comes a week after 49 inmates were killed in a riot at another penitentiary.