By Philip Pullella and Gabriel Stargardter
Pope Francis is heading to the once notorious Mexican border city of Ciudad Juarez, a major migrant crossing on the US doorstep where grisly drug violence has killed thousands, to pray for those who risk their lives migrating north.
Once one of the world's deadliest cities, Ciudad Juarez remains a violent magnet for Mexican, Central American and Asian migrants looking to jump across the US border.
Francis' visit, and his focus on the plight of migrants who risk murder, rape and extortion in their quest to reach the United States, is a symbolic critique of the anti-immigrant rhetoric of candidates for the 2016 Republican presidential nomination.
Billionaire Donald Trump has surged ahead of his rivals with his message that Mexico is "killing" the United States with cheap labour, while sending over "criminals, drug dealers (and) rapists." He has also called for a border wall to be built.
Trump last week dubbed the Pope "a very political person", saying he believed the Mexican government had put him up to the border visit.
"To suggest that the Pope is an instrument of the Mexican government, no. That is very strange indeed," said Father Federico Lombardi, the Vatican spokesman, shortly before the pontiff was due to arrive in Ciudad Juarez.
"The Pope always speaks of the problems of immigration. If Mr. Trump were to come to Europe he would see that the Pope has said the same things about immigration to the Italians, the Germans, the French and the Hungarians."
The Pope will be driven to the fence that separates Mexico from the United States, and will celebrate Mass just 73 metres from the crossing.
The Pope's arrival in Juarez comes as the number of Central American children and families apprehended at the border rises, in a spike reminiscent of a 2014 flood of migrants that created a major political headache for US President Barack Obama.