July 27th, 2013
04:58 PM ET
By Shasta Darlington and Eric Marrapodi, CNN
Rio de Janeiro (CNN) - A massive crowd packed Copacabana Beach late Saturday for a prayer service with Pope Francis.
The pope addressed Catholic pilgrims who came for the weeklong World Youth Day celebration.
Organizers said 3 million attended the service. They cited the mayor's office and police for the crowd estimates, which nearly doubled the 1.6 million who came here for a 2006 Rolling Stones concert and matched the 3 million who came for the 2000 millennium celebration.
CNN could not verify the estimate.
As he drove to the stage in the popemobile, Francis was pelted with T-shirts, banners and flags from the increasingly adoring crowds being held back by barricades on both sides of the street.
The pope told young people in attendance that they were the "protagonists of the future."
"In your young hearts, you have a desire to build a better world," Francis said. "I have been closely following the news reports of the many young people who throughout the world have taken to the streets in order to express their desire for a more just and fraternal society."
He said protesters should be motivated by the values of the Gospel and to overcome apathy with "a Christian response to the social and political concerns present in their countries."
The prayer vigil had been scheduled for a field 30 miles outside of the city at "Campus Fidei," or faith field, but organizers had to move events to Rio because of flooding.
"Today too, as always, the Lord needs you, young people, for his church," the pope said. "You are called to build a more beautiful church and a better world."
Many camping overnight on beach
Flags from around the world dotted the beach and marked off territories for the pilgrims with their sleeping mats who planned to stay overnight on the beach in anticipation of a final Mass on Sunday with the pope to close out the event.
"This is what the church needs now. He just brought so much new energy and so much joy," said Yana Slovakia who was attending with her church St. Anne Catholic Community outside Houston, Texas.
"In a church that has experienced much hemorrhaging of membership, tonight is a clear indication that young people are still willing to consider this Church their home," said the Rev. Thomas Rosica, a Canadian priest who has been working with the Vatican communications team.
"Just as John Paul II's election sent high voltage shocks into the Communist regime and eventually brought it down, I think that Francis' election has also sent some serious jolts deep into the shallow sects that have caused some hemorrhaging of the church in this part of the world."
Earlier, in a stern speech delivered to Brazilian bishops on Saturday, Pope Francis chastised the church for losing many of the faithful to other denominations, saying they must work harder to be close to the flock and employ a "grammar of simplicity."
"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 told the 300 bishops gathered for lunch.
The Vatican called it Francis' longest and one of his most important speeches to date. And it sets the tone for what can be expected from the pontiff's strategy for the Catholic Church as a whole.
Pontiff urges priests to get out of the office
Francis reiterated a familiar message, urging priests and higher-ups to get out their parishes and reach out to the people, especially those pushed to the margins of society.
Brazil has more Catholics than any other country, but the number has eroded in recent decades. According to a 2010 census, just 65% of the total population of 191 million considered itself Catholic, down from 74% in 2000.
In the same period, the percentage of Brazilians who considered themselves evangelicals grew from 15% to 22%.
On Saturday, Francis reflected on the causes.
"Perhaps the church appeared too weak, perhaps too distant from their needs, perhaps too poor to respond to their concerns, perhaps too cold, perhaps too caught up with itself, perhaps a prisoner of its own rigid formulas," he said.
"Perhaps the world seems to have made the church a relic of the past, unfit for new questions; perhaps the church could speak to people in their infancy but not to those come of age."
The pope arrived in Rio de Janeiro on Monday for the run-up to World Youth Day.
Before meeting with the bishops, he presided over Mass at Rio de Janeiro's Cathedral and then met with politicians and leaders of Brazilian society at the Municipal Theater.
The unpredictable pontiff even placed an indigenous headdress on his head briefly during the encounter.
Friday night, an estimated 1.5 million people turned out and the pope lead them in prayer.
Presidents and heads of state from across Latin America were scheduled to attend the final Mass, at which the pope is scheduled to announce the location of World Youth Day 2015.
CNN Vatican analyst John L. Allen Jr. contributed to this report.
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The CNN Belief Blog covers the faith angles of the day's biggest stories, from breaking news to politics to entertainment, fostering a global conversation about the role of religion and belief in readers' lives. It's edited by CNN's Daniel Burke with contributions from Eric Marrapodi and CNN's worldwide news gathering team.