Pope Benedict XVI lamented modern society's "amnesia" about God as he travelled Friday to a famed Spanish monastery on the second day of his four-day visit for the Roman Catholic Church's world youth festival.

Several hundred young nuns cheered, waved flags and performed the "wave" at El Escorial monastery as they waited for Benedict inside a courtyard of a 16th-century complex, a UNESCO world heritage site about 50 kilometres northwest of the capital of Madrid.

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Pope Benedict XVI, centre, arrives at San Lorenzo del Escorial monastery in Madrid on Friday, part of a four-day visit for the Roman Catholic Church's World Youth Day festivities. ((Arturo Rodriguez/Associated Press))

Benedict told them their decisions to dedicate their lives to their faith was a potent message in today's increasingly secular world.

"This is all the more important today when we see a certain eclipse of God taking place, a kind of amnesia which albeit not an outright rejection of Christianity is nonetheless a denial of the treasure of our faith, a denial that could lead to the loss of our deepest identity," he said.

Benedict's main priority as Pope has been to try to reawaken Christianity in places like Spain, a once staunchly Catholic country that has drifted far from its pious roots.

He has travelled to Spain three times as Pope, an indication that he views it as the key battleground as he tries to remind Europe of its Christian heritage and the place he believes God should still have in everyday life.

'This is a moment for unity and reflection'

That he chose to deliver his message in El Escorial is significant: The massive granite structure constructed by King Philip II in 1559 was his seat of power over a vast empire whose overwhelming international concern was defending the Catholic faith from what it considered the threat of Protestantism and the Reformation.

The building acted much like the White House and the Pentagon at the height of Spain's international power, throwing its weight and organizational ability behind the Vatican.

"This is a moment for unity and reflection; it's a spiritual awakening," said Sister Maria Sandoval, a 58-year-old nun who travelled from Medellin, Colombia, for the event.

Benedict later met with university professors at El Escorial, saying he was reminded of his own days as a young theologian at the University of Bonn in the years after World War II.

"At the time, the wounds of war were still deeply felt and we had many material needs," the 84-year-old German pontiff recalled. But he said those needs were taken care of by the "passion" he and his colleagues felt to respond to the heady questions about life and the search for truth posed by their students.

He urged the professors to not just educate today's young in the "technical ability" they may need to enter the workforce, but to guide them in pondering those loftier questions which he said embrace "the full measure of what it is to be human."

"We know that when mere utility and pure pragmatism become the principal criteria, much is lost and the results can be tragic," he said, adding: "From abuses associated with a science which acknowledges no limits beyond itself, to the political totalitarianism which easily arises when one eliminates any higher reference than the mere calculus of power."

His speech, dense and professorial as befits both speaker and audience, received a thunderous applause from the academics, who were decked out in their colorful tasseled caps and gowns in El Escorial's basilica.

His meetings, and a private audience with members of Spain's royal family Friday morning, came after a second relatively minor night of clashes between riot police and protesters opposed to his visit and the church's World Youth Day.

Four protesters suffered light injuries after riot police wielding truncheons forced several hundred people to leave Madrid's central Sol plaza on Thursday night, sending them scurrying through side streets with officers in pursuit. No arrests were made, said a police spokeswoman who spoke on condition of anonymity in keeping with department policy.

The demonstration was much smaller than a protest by 5,000 people on the eve of the Pope's visit  for the youth festival. It also ended in violence when a smaller group clashed with police in Sol, resulting in more injuries and detentions.

Protesters angry about $72M youth event tab

Protesters have used Sol since May as the epicentre of their rage against Spain's political establishment, the government's anti-austerity measures and unemployment of nearly 21 per cent, a eurozone high.

They also are angry about the $72-million US tab for staging World Youth Day as Spain struggles economically.

The church says the weeklong festival is being paid for by participants, donors and the church, but pilgrims are staying for free in government buildings and getting deeply discounted subway and bus tickets, while public transport fees were raised significantly for everyone else this month.

As he arrived Thursday, Benedict offered words of encouragement to young people facing precarious futures because of the economic crisis, calling for policy makers to take ethical considerations that look out for the common good into account when formulating economic policy.

Late Friday, Benedict participated in a re-enactment of the death and crucifixion of Christ, a mainstay of World Youth Days. Meditations read out during the solemn service included prayers for young victims of war, poverty and sexual abuse, though there was no explicit reference to sexual abuse by priests.