AFP -

Pope Benedict XVI headed to Madrid on Thursday for a million-strong Catholic youth festival after Spanish riot police swung batons in clashes with anti-Church protesters over the cost of the visit.

Benedict flew out of Rome around 9:30 am (0730 GMT), hours after thousands took to the streets of Spain's capital to decry the official 50 million euro bill for the six-day World Youth Day festival at a time of economic hardship.

After an official greeting in Madrid-Barajas airport he will enter the city driving through the streets in his transparent popemobile to the Vatican ambassador's residence, chased by pilgrims running a relay race.

In the evening, the papal cortege heads to the emblematic Plaza Cibeles in central Madrid for a welcoming ceremony. The Spanish air force will fly over, drawing the colours of the Vatican and Spanish flags in the sky.

Late into the night on the eve of the 84-year-old pontiff's arrival, anti-papal protesters and hundreds of young Roman Catholics hurled insults at each other in Madrid's central square Puerta del Sol.

Lines of riot police separated the two sides.

Protesters chanted: "God yes, Church no"; "Not with my taxes"; "We are not the pope's youth"; and "I am a sinner, sinner, sinner."

Some taunted the faithful shouting: "Nazi, nazi." In an interview before he was elected pope in 2005, then Cardinal Joseph Ratzinger said he was an unwilling participant in the Hitler Youth movement.

"Long live the pope", "We are the pope's youth," cried the pilgrims, some of whom sang "hallelujah" and sat on the ground and prayed.

One protester against the papal visit, 18-year-old Ignacio who declined to give his full name, bled from the nose. "I was in the demonstration with my father and one of the Catholics punched me in the face," he said.

Police moved in with batons swinging to dislodge the anti-pope demonstrators from Puerta del Sol.

They arrested seven people for "assaults on people in authority," a national police spokeswoman said Thursday.

Eleven people were lightly injured, she said.

More than 100 protest groups took part, uniting many causes, including those seeking a change in the Church's attitude to gay rights and those fighting for a clearer separation of Church and state.

The outcry that has struck a chord with many -- including some priests -- is over the official 50.5-million-euro ($73-million) price tag, excluding the cost of police and security, of the Madrid celebrations.

Some protesters argue the real cost to taxpayers is more than 100 million euros including the policing, cleaning and discounts such as half-price bus fares.

Organisers say most of the cost will be covered by a registration fee from the assembled pilgrims, and the celebration will be a massive tourist boost for Spain.

The Spanish government is currently battling to meet tough deficit reduction targests at a time when youth unemployment is running at more than 45 percent.

The clashes were an ugly scene-setter to the arrival of the pope for the final four days of the rock festival-style celebration, luring more than a million pilgrims from around the world.

Traffic has been banned from much of the city centre.

Huge speakers blare out pop music throughout the day in the city as hundreds of thousands of faithful fans in floppy hats bake in the August heat.

The Church has opened 200 white confessionals in the form of boat sails along the main thoroughfare through Madrid's Retiro park.

The pope will hold a "Prayer Vigil" on Saturday evening at an airbase southwest of the capital, where the pilgrims will spend the night on an esplanade the size of 48 football pitches.

Pope Benedict will celebrate mass there on Sunday morning at a white altar almost 200 metres (660 feet) long in front of a wave-shaped stage and under a giant parasol "tree", made of interwoven golden rods.