Pope Benedict XVI warned yesterday of the return of a 1930s-style “aggressive” anti-clericism in Spain and called on Europe to rediscover its Christian roots.

The 83-year-old pontiff, fighting a slide away from the Roman Catholic Church’s core beliefs, recalled the years when Republican forces killed priests and nuns and burned churches before and during the Spanish Civil War.

“Spain saw in the 1930s the birth of a strong and aggressive anti-clericism,” he told reporters on the plane from Rome to Spain’s holiest city, Santiago de Compostela, ahead of a giant open-air mass in front of thousands of pilgrims.

“The clash between faith and modernity is happening again and it is very strong today,” he said as he embarked on a two-day visit to bolster the Church against an onslaught of social change in Spain and Europe.

Benedict waved as he emerged from a fog-enshrouded Alitalia plane for a visit aimed at upholding the Church’s core beliefs against a drift towards easy divorce, abortion and gay marriage.

His scarlet papal mozetta laced at the neck over his white cassock, Benedict leant from the window of his popemobile to kiss babies wrapped in blue or pink blankets and carried by officials into his arms.

Pilgrims lined the 11-kilometre route from the airport as the papal cavalcade made its way to the centre of Santiago de Compostela and its landmark cathedral of St James.

Many cheered and waved flags or threw yellow and white confetti, the colours of the Vatican flag.

The pope said the pilgrim routes to Santiago de Compostela created a spiritual unity across Europe, and he recalled how Pope John Paul II had called for the continent to reinvigorate its Christian roots.

“I too wish to encourage Spain and Europe to build their present and to project their future on the basis of the authentic truth about man, on the basis of freedom which respects this truth and never harms it,” he said.

A giant carpet of flowers lay at the entrance to the 12th century cathedral, where Benedict prayed alone for a few minutes in the brilliantly decorated room holding the tomb of what are believed to be the remains of St James the Apostle.

Then he rose to his feet to embrace a painted statue of the first century saint clutching a staff.

Santiago, which Benedict said was one of the three holiest pilgrimage sites after Jerusalem and Rome, has been a magnet for pilgrims since the discovery of the remains in the Middle Ages.

The pope was to celebrate mass outside on the city’s medieval cobbled Plaza Obradoiro in front of around 7,000 people, a highly symbolic venue from which to launch a drive to restore Christian values.

St James, later to be known as the Slayer of the Moors, became a symbol to rally Christian Spain, then pinned down by the Muslim Moors to the northern strip of the peninsula.

Benedict gently chided Christians who hike the Way of St James pilgrimage just for the view. “To go on a pilgrimage is not simply to visit a place to admire its treasures of nature, art or history,” he said.

The pontiff had to spend the day in Santiago de Compostela before heading to Barcelona in the evening and then today sanctifying Antoni Gaudi’s unfinished masterpiece, the Sagrada Familia church.

Children as young as 10, elderly men and women, monks, nuns, priests and some in wheelchairs, queued in the cold outside the vast square from before dawn yesterday.

“I’m not at all tired, I’m emotional,” said Jose Antonio, a 30-year-old Spanish monk who had stood at the head of the queue since 6.30 p.m. on Friday.

The Church was an all-powerful presence in the dictatorship of General Francisco Franco, who defeated the Republicans in the Civil War and died in 1975, but with democracy came an end to restrictions on politics, behaviour and sexual mores.

Under the Socialist Party of Prime Minister Jose Luis Rodriguez Zapatero the country has gone much further, allowing gay marriage, speedier divorce and easier abortions.

Of particular concern to the Church, a law passed four months ago lets women have abortion on demand up to 14 weeks of pregnancy, and in case of risk to the life and health of the mother until 22 weeks.

Girls of 16 and 17 can get an abortion without their parents’ consent if they face a risk of family violence, threats, or pressure.

In eight years, the proportion of Spaniards who describe themselves as Roman Catholic has dropped to 73 per cent from 80 per cent and those attending weekly mass to 13 per cent from 20 per cent.

This is Benedict’s second visit to Spain. He is scheduled to return next year to preside over World Youth Day festivities in Madrid.