Eight demonstrators have been arrested and 11 people injured in clashes between riot police and protesters in Madrid opposed to a visit by the Pope.
Police said two of the injured were police officers.
Violence broke out after a march by thousands of people angry that - among other things - taxpayer money is being spent on the four-day visit by Pope Benedict XVI for a festival called World Youth Day.
Police wielding truncheons charged demonstrators several times in Madrid's Puerta del Sol plaza, scene of anti-establishment rallies in recent months.
Meanwhile, a chemistry student working as a volunteer for Benedict XVI's visit was arrested on suspicion of planning a gas attack targeting the protesters.
A police official said the suspect is a 24-year-old Mexican specialising in organic chemistry. She would not say whether investigators believe the man was actually capable of carrying out a gas attack, and did not know if he had chemicals that could have been used to assault the protesters.
The detainee was identified by the Mexican Embassy in Madrid as Jose Perez Bautista. A statement said he was from Puebla state, near Mexico City.
This will be the third time the pontiff has visited Spain since his papacy began in 2005, and the second in less than a year.
But the Pope faces a country whose economy is in a shambles with jobless youths filled with rage and frustration.
Many Spaniards take issue with the hefty cost of the visit. The €50m tab for setting up everything from giant screen TVs to portable toilets and confession stalls has hit a raw nerve even among some churchgoing Catholics and priests.
The Pope has denounced the profit-at-all-cost mentality that he says is behind Europe's current economic crisis.
Speaking as he travelled to Madrid, he said morals and ethics must play a greater role in formulating economic policy.
The Pope said: "Man must be at the centre of the economy and the economy must not be measured only by the maximisation of profit but according to the common good."
His plane touched down in Madrid shortly before noon to a crowd of hundreds of young pilgrims cheering and waving mainly Spanish flags.