Thousands of protesters have rallied in the Spanish capital,

thousands of protesters have rallied in the Spanish capital, Madrid, against the cost of a visit by the Pope, hours before he is due to arrive.

A number of the demonstrators clashed with police after marching to the central Sol square chanting slogans.

Earlier, Spanish police arrested a Mexican student for allegedly plotting to gas the anti-Pope rallies.

The trip comes at a time of economic hardship in Spain, although organisers say the pilgrimage is self-funded.

Up to a million pilgrims from across the world have gathered in the city for the six-day World Youth Day (WYD 2011) festival, which culminates with an open-air mass on Sunday celebrated by Pope Benedict XVI.

The Pope will arrive at Madrid's Barajas airport on Thursday morning and hold meetings with young people at WYD 2011 later in the day.

More than 100 groups opposed to the Pope's visit planned to protest on Wednesday evening.

They include those who belong to the 15-M "indignant" movement - who oppose the government's austerity drive - as well as gay rights groups and others who oppose aspects of Catholic teaching.

"We are not angry about the Pope's visit, which some will agree with and others won't, but rather over the financing of it with public money, especially at a time when many services are being cut because it's necessary to curb government spending," 15-M said in a statement.

Protesters walked into the old city centre shouting "Nothing for the Pope from my taxes".

"It is costing a lot of money for the Spanish state which is going through a bad period," one protester, 55-year-old Rosa Vazquez, told Reuters news agency.