Hackers temporarily blocked the official World Youth Day site Thursday just as Pope Benedict XVI arrived in Madrid to preside over the festivities, a spokeswoman for the event said.

The site (www.madrid11.com) was taken down for about an hour at around midday (1000 GMT) and the event's computer experts were trying to determine the source of the attack, the spokeswoman said.

"We received in recent days anonymous warnings by email that our site would be attacked. The priority now is to ensure everything is up and running, then we will decide if we will file a police complaint," she said.

The pope arrived in Madrid on Thursday for the final four days of the Roman Catholic festivities and the organisers had planned to provide live coverage on the web.

On the previous evening, thousands of protesters took to the streets of Spain's capital to decry the official 50 million euro ($72 million) bill for the six-day event at a time of economic hardship.

The price tag includes 2.1 million euros spent on the event's computer system.