German authorities have increased security at Cologne Cathedral following warnings of a possible attack plot.
Worshippers attending the Christmas Eve service were told to arrive early and avoid bringing large bags with them.
The city's police chief said information pointed to threats over New Year's Eve rather than Christmas, but security would be boosted nonetheless.
There have been heightened fears of attacks in several European countries over the festive period.
Authorities used sniffer dogs to search Cologne cathedral on Saturday evening but no traces of explosives were found.
"We are doing everything we can to ensure the safety of visitors," police chief Michael Esser said.
Authorities in Germany, Austria and Spain have said radical Islamists could be planning attacks around Christmas or New Year.
On Saturday, four people were arrested in Austria. Media reports say the suspects may have links with Islamic State Khorasan Province (IS-K), an offshoot of the Islamic State group.
It is not clear whether the arrests were connected to the alleged plot in Cologne.
Dozens of police officers have been deployed near Vienna's St Stephen's cathedral. There was no danger of an immediate attack in the area, the interior ministry said on Sunday.
Earlier this month, seven people were detained in Germany, Denmark and the Netherlands on suspicion of planning attacks.
On 5 December European Home Affairs Commission Ylva Johansson said the EU was facing a "huge risk of terrorist attacks" over the holiday period.
