Pope in Spain: Pilgrims in Madrid's protest square

Pope Benedict XVI is in the Spanish capital, Madrid, for four days of events 

The BBC's Sarah Rainsford in Madrid said tension had increased as the protest attempted to cross Puerta del Sol square, the site - and symbol - of months of mass protests against unemployment and austerity.

When police tried to clear the square, officers in riot gear clashed with demonstrators - some of them throwing bottles and water.

Spain is going through its worst economic crisis in decades, with its 21% unemployment rate the highest in the EU.

Organisers say they believe the youth festival will generate about 100m euros (£88m; $144m) for the Spanish economy "at zero cost to taxpayers", but critics estimate the event will cost a similar sum.

The government has declined to give a figure for the costs.

Meanwhile, a Mexican chemistry student who was accused of planning to attack an anti-Pope march with "asphyxiating gases and other chemical substances" has been released on bail.

Jose Perez Bautista's government-appointed attorney, Antonio Ortiz, said police had not seized any chemicals from his client, who had not intended to stage an attack.

He said the Mexican's online messages that had led police to arrest him were merely "a joke in bad taste".