Al-Qa'eda sympathisers are threatening to attack the Euro 2008 football tournament next month, according to Swiss police.
The championships, which begin on June 7 and are hosted by Switzerland and Austria, are in a "terrorist danger zone" and police are monitoring closely a number of internet chat forums linked to the terror group.
Juerg Buehler, a security expert with the Swiss federal police, said: "The Euro 2008 tournament is a potential target cited by the Islamist terrorist network."
Messages have been posted on "sos minbar" and "As-Sahab", two Islamist websites used by al-Qa’eda and its followers.
Mr Buehler said: "We are taking these threats seriously. We are on alert and we are following these jihad forums very closely. It is through these that Osama bin Laden’s agents awaken dormant cells.
"The situation is serious even if it is frustrated people hiding behind these sites."
According to the official, one site said: "Let’s transform the two most secure countries in Europe into hell, like the hell in Iraq and Afghanistan."
Other messages said: "The hour has come for fighters of the faith. They must make themselves heard" and "Austria must withdraw from Afghanistan".
A Swiss police spokesman Guido Balmer stressed that there was "no concrete evidence" that an attack was being planned but said all threats were taken seriously.
He said: "The tournament is a potential target, as are the Olympics and other big sporting events. They are in terrorist danger zones. But it is wrong to say that we have firm intelligence of a specific plot. We don’t have concrete evidence of preparation or planning. There are many discussions on Islamist websites which we are monitoring.
"A major event such as Euro 2008 could well be a rewarding goal from the perspective of terrorists."
More than five million people are expected to attend Euro 2008.
Sixteen teams are competing and the first match is in the Swiss city of Basle. There are eight venues in Switzerland and four in Austria.
Switzerland has been widely accused of Islamophobia in the Muslim world, notably because of opposition to the construction of mosque minarets in several Swiss cities.
Austria could also be a target because of the presence of its troops in the international force in Afghanistan.
Counter-terrorism experts say Islamist forums regularly carry threats of attacks which have to be studied, though few are to be taken seriously
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