Cologne police chief Michael Esser said on Saturday that Cologne Cathedral would be closed and sniffer dogs would be brought in to search the building after the evening mass service.
“Even though the information relates to New Year’s Eve, we are from this evening doing everything we can to ensure the safety of visitors of the cathedral on Christmas Eve,” Esser said in a statement.
Visitors on Sunday will have to undergo security checks before entering the cathedral.
German newspaper Bild reported that authorities in Germany, Austria and Spain have all received indications that an extremist group was plotting attacks in Europe, with targets possibly including Christmas masses in Cologne, Vienna and Madrid.
On Saturday, special forces in Vienna and Germany arrested a number of suspects, Bild also reported.
Austrian police said in a statement they were boosting security for churches and Christmas markets due to the heightened state of alert.
“Given that terrorist actors throughout Europe are calling for attacks on Christian events, especially around December 24, the security authorities have taken the corresponding protection measures in public spaces,” the police said in a statement.
Earlier this month, Spain’s Interior Ministry announced it would ramp up security measures for the Christmas holiday period.
Cologne Cathedral, which was designated a UNESCO World Heritage Site in 1996, is one of Germany’s most popular tourist attractions, attracting more than six million visitors each year.