Borussia Dortmund said some of their fans will not be able to attend Wednesday's Champions League match at Atalanta after being denied entry to Italy while others have been visited by the police at their accommodation.
"Borussia Dortmund are currently unaware of any detailed background information regarding the measures taken by the authorities," the club said in a statement.
"While Borussia Dortmund understand the need for security-based risk assessments for football matches, the club is more than surprised by the scope and scale of the police measures taken."
Dortmund lead 2-0 heading into the second leg. The winners will meet with either Arsenal or Bayern Munich in the round of 16.