Pope Francis, starting Holy Week services leading to Easter, has urged young people to keep shouting and not allow older generations to silence their voices or anesthetise their idealism.
Francis spoke a day after hundreds of thousands of young Americans and their supporters answered a call to action from survivors of last month's Florida high school massacre and rallied across the United States to demand tighter gun laws.
He did not specifically mention the demonstrations.
The pontiff led a long and solemn Palm Sunday mass before tens of thousands in St Peter's Square, many of them young people there for the Catholic Church's World Day of Youth.
Carrying a woven palm branch known as a "palmurello," Francis urged the young people in the crowd not to let themselves be manipulated.
"The temptation to silence young people has always existed," Francis said in the homily.
"There are many ways to silence young people and make them invisible. Many ways to anesthetise them, to make them keep quiet, ask nothing, question nothing.
"There are many ways to sedate them, to keep them from getting involved, to make their dreams flat and dreary, petty and plaintive," he said.
"Dear young people, you have it in you to shout," he told young people, urging them to be like the people who welcomed Jesus with palms rather than those who shouted for his crucifixion only days later.
"It is up to you not to keep quiet. Even if others keep quiet, if we older people and leaders, some corrupt, keep quiet, if the whole world keeps quiet and loses its joy, I ask you: Will you cry out?"
Young people in the crowd shouted, "Yes!"
Palm Sunday marked the start of a hectic week of activitiesfor the pope.
On Holy Thursday he is due to preside at two services, including one in which he will wash the feet of 12 inmates in a Rome jail to commemorate Jesus' gesture of humility towards his apostles the night before he died.
On Good Friday, he is due to lead a Via Crucis (Way of theCross) procession at Rome's Colosseum.
On Saturday night he leads a Easter vigil and on Easter Sunday he delivers his twice-yearly "Urbi et Orbi" (to the city and the world) message.