Filipinos are famous for practising a passionate brand of Catholicism and they turned out in a celebratory mood that defied the gloomy skies, determined to see the charismatic 78-year-old pontiff.
Francis made an exhilarating entrance to a Manila bayside park for the mass aboard a "popemobile" that was styled after the nation's iconic, flamboyant and much-loved "jeepney" minibus.
Dressed in a plastic yellow poncho, he waved and smiled to wildly cheering crowds, stopping repeatedly so he could lean over barriers and kiss babies, before reaching the sea of believers at Rizal Park.
But even the pope was stunned at the size of the crowd which he looked out on from the stage.
"I cannot fathom the faith of the simple people," Francis said, according to the Archbishop of Manila, Cardinal Luis Antonio Tagle, who acted as the pontiff's chaperone.
Six million turned out to see the pope at the park and along motorcade routes, the head of Manila's planning agency, Francis Tolentino, told AFP, adding this was based on calculations done with the police.
Vatican spokesman Federico Lombardi said today's crowd could even reach seven million. However he did not have any firm assessments to base this on.
Perla Ponce, 76, a retired nurse who travelled hundreds of kilometres from her home in the southern Philippines, said she felt "euphoric" to have seen the pope and been a part of the mass.
"You can't describe the feeling. It's just like you have met Jesus Christ himself," a rain-soaked but beaming Ponce told AFP.