The pope, making his first foreign trip since his election in March, told the crowd on Copacabana Beach, better known as a venue for rock concerts, that Jesus is the champion of the poor and oppressed, USA Today reported.
"On the cross, Jesus unites himself to the silence of the victims of violence," Francis said.
Francis, a native of neighboring Argentina, was known for his simple lifestyle when he was Cardinal Jorge Mario Bergoglio in Buenos Aires. The young pilgrims who came to Rio de Janeiro for World Youth Day praised him with some calling him a good change from his predecessor, Pope Benedict XVI.
"He's humble and doesn't go on and on," Sarah Cadden from Ireland said.
The mass was moved to Copacabana because heavy rain had turned the original site to mud.
Francis also met with young prisoners Friday morning, Catholic News Service reported.