The Pope greeted the pilgrims from his Popemobile, stopping to kiss babies, before addressing the crowds from an impressive front stage during the vigil on the final night of World Youth Day.
In a reference to the thousands of young people who have been protesting for change in Brazil in recent weeks, the Pope said they were "the ones who want to be actors of change".
He appealed to young people to be active in responding to the needs of the world.
"Please don't let others be actors of change," he said. "Keep overcoming apathy and offering a Christian response to the social and political concerns taking place in different parts of the world."
Throughout the week, the Pope has used his visit to Brazil to encourage the young faithful to be confident in living out their faith.
However, he also used his first international trip abroad to challenge the decline in Catholic membership and urge bishops and clergy to rethink how they engage with the world.
"We cannot keep ourselves shut up in parishes, in our communities, when so many people are waiting for the Gospel," he said in an address at Rio's cathedral.
"It's not enough simply to open the door in welcome, but we must go out through that door to seek and meet the people."
He also suggested that "intellectualism" and "rigid formulas" were a turn-off for people who just wanted to understand God.
"At times we lose people because they don't understand what we are saying, because we have forgotten the language of simplicity and import an intellectualism foreign to our people," he said.
"Without the grammar of simplicity, the church loses the very conditions which make it possible to fish for God in the deep waters of his mystery."
The Pope was due to attend a Mass at Copacabana beach on Sunday to close World Youth Day.