Authorities have imposed border controls until July 4 in an effort to prevent disturbances during the Euro 2004 soccer championships that begin June 12. EU citizens and Americans must show passports on arrival. Mobile patrols have been set up on highways rather than reopening old border posts. The championships have been plagued by hooligan violence in the past, and hundreds of thousands of foreign soccer fans are expected to attend. Police from some countries, including the U.K. and Switzerland, are sending officers to help the Portuguese keep an eye on troublemakers.