Security measures have been strengthened in crowded areas over the holiday period across Europe.
ADVERTISEMENTSecurity has been stepped up around Christmas celebrations in the Vatican, Germany and Austria over attack concerns.
Sightseeing visits were barred at Germany's landmark cathedral in Cologne and Christmas Eve worshippers faced security checks to get into midnight Mass on Sunday.
The moves came amid intelligence warnings indicating a potential attack.
However, a top security official urged people not to shy away from holiday celebrations out of fear.
Churchgoers attended multiple services at the cathedral in Cologne despite the ban on sightseeing visits, a day after police descended on the cathedral and searched it with sniffer dogs.
With several dozen officers on duty outside, Auxiliary Bishop Rolf Steinhaeuser greeted those attending what he said was “probably the most secure church service in all of Germany.”
In Austria, police said they also were stepping up security around Vienna's churches and Christmas markets, apparently responding to the same intelligence about a potential threat.
They did not give further information, but the DPA news agency reported without citing a source that the threat was from an Islamic extremist group.
Police in Cologne said they were taking precautions over Christmas even though the information they had was for an attack on New Year's Eve.
“Even if the reference was to New Year's Eve, there are a lot of people in the area around the cathedral, today is Christmas Eve midnight Mass, it is one of the most visited cathedrals, the main train station is nearby,” police spokesman Wolfgang Baldes said while standing in front of the cathedral.
“There are a lot of people, and that's why we said that if there was an indication, we would do everything necessary to protect people,” he added.
At the same time, Herbert Reul, interior minister for the state of North Rhine-Westphalia, appealed to the public not to stay away.
“Caution is advised," he said, but “we are not defenceless. Our agencies are using all available information to protect us as well as possible.”
“My appeal is, go to church, celebrate Christmas. Fear is the currency of terrorists. We should not make it more valuable,” Reul said.
The European Union’s home affairs commissioner, Ylva Johansson, warned on December 5 that Europe faces a “huge risk of terrorist attacks” over the Christmas holidays due to fallout from the war between Israel and the Palestinian militant group Hamas.