Pope Benedict XVI encouraged thousands of young people gathered for World Youth Day in Spain to avoid temptation and non-believers who think they are 'god.'

"There are many that, believing they are god, gods, think they have no need for any roots or foundations other than themselves, they would like to decide for themselves what is true or isn't, what is right and wrong, what's just and unjust, decide who deserves to live and who can be sacrificed for other preferences, taking a step in the direction of chance, without a fixed path, allowing themselves to be taken by the pulse of each moment, these temptations are always there, it's important not to succumb to them," the Pope said during his first speech to the pilgrims.

"Taking a step in the direction of chance, without a fixed path, allowing themselves to be taken by the pulse of each moment, these temptations are always there, it's important not to succumb to them."

The four-day World Youth Day event is held once every three years and is expected to attract one million pilgrims from 193 countries.

Pope Benedict XVI waves from his Popemobile as he crosses the city of Madrid on his way to Cibeles central square, Thursday. Daniel Ochoa de Olza/Associated PressBut the event has its critics who say too much money is being spent on the Pope's visit during a time when austerity measures are being imposed on citizens.

Spanish police say eight demonstrators were arrested and 11 people injured in clashes between riot police and protesters prior to the Pope's arrival in Spain.

A police official said Thursday that two of the people injured in the disturbances in Madrid were police officers.

Violence broke out Wednesday night after a march by thousands of people angry that taxpayer money is being spent on the week-long youth festival for things like security.

But others have a different view.

"I think that the outcome in the hope that it gives the soul and the good that it will bring to the world is worth the money, there is no comparison, and also with the economic factor, so much money is going to come out of it too because of the tourism it is bringing to this city," Sarah Shaw, who is visiting from the United States, told Reuters news agency.

The Pope was greeted with loud cheers during a visit to Cibeles square in Madrid, where thousands of young people had gathered to meet him.

The Pope smiled as he shook the hands of those who greeted him in Puerta de Alcala, as music played in the background. He seemed amused by horsemen who paraded in front of him. Security was tight around the Popemobile as he moved into the square, followed by a band of musicians.

Before his arrival in Madrid, the Pope told reporters on the papal plane that ethics must play a role in economic policy.

"The economy doesn't function with market self-regulation but needs an ethical reason to work for mankind," he said. "Man must be at the centre of the economy, and the economy cannot be measured only by maximization of profit but rather according to the common good."

Police charge at demonstrators

Earlier, police wielding truncheons charged demonstrators several times in Madrid's Puerta del Sol plaza, the site of anti-establishment rallies in recent months.

The police official spoke on condition of anonymity in line with department rules.

Earlier this week, a chemistry student working as a volunteer for the Pope's visit to Madrid was arrested on suspicion of planning a gas attack targeting protesters opposed to the pontiff's stay.

A police official said the suspect is a 24-year-old Mexican student specializing 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 actually 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 from Puebla state, near Mexico City.

With files from CBC