Organisers expect as many as 6 million people at the afternoon Mass, surpassing the 5 million record set by St John Paul II at the same Rizal Park in 1995.
Francis drew a huge crowd earlier on Monday when he addressed young people at Manila's Catholic university, coming close to tears himself when he heard two young children speak of their lives growing up poor and on the streets.
Francis has dedicated his four-day trip to the Philippines to the poor. He has denounced the corruption that has robbed them of a dignified life, visited with street children and travelled to eastern Tacloban to offer prayers for the survivors of the deadly 2013 Typhoon Haiyan that devastated one of the Philippines' poorest regions.
Francis ditched his prepared remarks again, speaking off the cuff to respond to four young people who spoke of their lives. The most poignant testimony came from little Glyzelle Palomar, age 12, who wept as she asked Francis why children, in particular, suffer so much. She spoke of children who are abandoned by their parents and end up in prostitution, but couldn't get through her remarks.

"Why is God allowing something like this to happen, even  to innocent children?" Palomar said through tears. "And why are there so few who are helping us?"
A visibly moved Francis said he had no answer. "Only when we are able to cry are we able to come close to responding to your question," Francis said.
"Those on the margins cry. Those who have fallen by the wayside cry. Those who are discarded cry," he said. "But those who are living a life that is more or less without need, we don't know how to cry."

A steady rain from the same tropical storm that forced Francis to cut short his visit to Tacloban yesterday fell on the crowd, but it didn't seem to dampen spirits of Filipinos who streamed into the capital for Francis' final day.

"I am not satisfied just seeing him on TV," said Rosalinda Kho, a devout Catholic who arrived before dawn outside Rizal Park with her daughter, Rosana, to score a place for the Mass.

"This is a once in a lifetime chance to see him in the flesh, even from afar." By the time the gates opened, the roads leading to the park were mostly closed. Many people camped out on tarpaulins on the ground where they had spent the night, surrounded by bags of food.

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