The championships in Switzerland and Austria start June 7 and Swiss federal police spokesman Juerg Buehler told La Liberte daily that the tournament "is a target cited by the Islamist terrorist network" on websites.

"We are following the situation very closely," he said.

He said similar threats were made before the World Cup in Germany in 2006, and that some attacks were planned against the rail network but were called off because security was considered too tight.

"We are prepared for every eventuality," he said.

A top official of European football's governing body UEFA said that arrests, never previously announced, were also made before the 2004 European championships in Portugal.

In a statement however, Swiss police said: "Any big event like Euro 2008 could be a target of choice for terrorists... but there are no concrete elements right now that would lean one to fear any specific danger."

No extremist group has given any indication that it will target the football championships, police added.

"It is clear however that how things develop in the coming weeks and months in global hotspots could have security repercussions at home," the statement said.