Pope John Paul II has urged young people not to be afraid to go "against the current" in his Palm Sunday address to crowds in St Peter's Square in Rome.
Cardinals line up to receive the papal blessing on Sunday
He spoke with some difficulty and often appeared weary at the start of an intense week of religious events, culminating in mass on Easter Sunday.
As well as concern over the health of the 83-year-old, who has Parkinson's, there are elevated security fears.
There have been warnings Italy and the Vatican could be targets of an attack.
Tens of thousands of young people packed into St Peter's Square for the Pope's address on Sunday, carrying palm fronds and olive branches.
The ceremony celebrates the start of Holy Week. Palm Sunday marks Christ's entry into Jerusalem the week before his crucifixion.
Test of faith
The Pope at first struggled to pronounce his words, but later spoke with more strength.
"Certainly, the message that the Cross communicates is not easy to understand in our day, where material wellbeing and comfort are proposed and sought after as priority values," he said.
"Do not be afraid to go against the current" he urged.
Palm Sunday is traditionally a day the Pope uses to remind followers - particularly the young - of the strength of their commitment to their faith.
Alert for trouble
The Pope appeared unconcerned by the heightened security alert, distributing Communion at the steps of the altar, and later touring the margins of the crowd in his popemobile.
But pilgrims were searched as they entered the square, and plainclothes police and Vatican security scanned the crowds as the ceremony proceeded.
Palm Sunday celebrations were also held in many other countries
Easter Sunday falls on the 11th of the month and various secret services have indicated that Italy and the Vatican could be particularly at risk.
The attacks on New York and Washington took place on 11 September 2001, and the Madrid train bombings on 11 March this year.
On Friday Italian police detained 161 immigrants, mainly North Africans, suspected of having links with Islamic extremist groups.
'Sowers of hate'
The Vatican has described the week of holy rituals as an "antidote" to the fears of a world grappling with war and violence.
Vatican newspaper L'Osservatore Romano ran an article about Sunday's celebrations next to photographs of the terror bombings in Madrid, Middle East violence and the Iraq war.
"The most insidious enemy of young people is called
fear," the paper said.
"An 'expert' of hope, like John Paul II, knows this well."
The pontiff, it said, invites youths to "ally themselves on Christ's side, and not with the sowers of hate, the mercenaries of terror, the prophets of death."