Security for the long Christmas weekend was heightened throughout Italy and at the Vatican on Saturday following the killing by police of the man believed to be responsible for the Berlin market truck attack.
As investigators sought to determine if Anis Amri had accomplices in Italy, and associates of the 24-year-old were arrested in his home country of Tunisia, national security officials were taking no chances.
Rome authorities banned vans or trucks from entering the city centre and anti-terror police wearing masks and wielding machine guns set up roadblocks on routes leading to famous tourist sites or areas where crowds traditionally gather.
At the Vatican, where Pope Francis was due to celebrate Christmas Eve Mass in St. Peter's basilica on Saturday evening, police cars and military jeeps stood about every 100 metres (yards) along streets leading to the Vatican.
Security was also stepped up in central Milan and other Italian cities, particularly near major churches where faithful were attending Christmas services.
After reconstructing Amri's movements since he drove a truck through a festive market in Berlin on Monday, killing 12 people, police are investigating whether he was seeking shelter from comrades in Italy or was en route to another country.
The town where Amri was killed, Sesto San Giovanni, is home to a sizeable Muslim community and is a departure point for buses to southern Italy, eastern Europe and the Balkans.
Amri had travelled undetected to Italy from Germany via France, taking advantage of Europe's open-border Schengen pact. He was shot dead in the town on the outskirts of Milan early on Friday after he pulled a gun on police during a routine check.
In a video released on Friday after his death, he is seen pledging his allegiance to militant group Islamic State (IS) militant leader Abu Bakr al-Baghdadi.
On Saturday, Tunisian security forces arrested three suspected militants, including Amri's nephew, who had been in touch with Amri by social media messaging.
Spain's interior minister Juan Ignacio Zoido said its intelligence services were investigating a possible connection via Internet between Amri and a Spanish resident on Dec. 19.
Amri originally came to Europe in 2011, landing with other migrants on the island of Lampedusa, and spent four years in an Italian jail for trying to set a school on fire in Sicily.
German authorities have complained they were unaware of Amri's criminal past.
"Convicted criminals from all countries need to be listed in a European database so that we know when and where they are when they cross our borders or ask for asylum," German Interior Minister Thomas de Maiziere told Bild am Sonntag newspaper.
Italians have been moved by the story of 31-year-old Fabrizia di Lorenzo, who survived two heart operations only to be killed in the truck attack in Berlin, where she was living.
President Sergio Mattarella went to Rome's Ciampino military airport on Saturday morning to grieve with her family as her body was returned from Germany. The arrival was broadcast live on Italian television.
Concerns about potential extremist violence also clouded the Christmas weekend in France, where the authorities said more than 91,000 policemen and soldiers would be deployed, with additional security at churches.
Fears of attacks by Islamist militants are running high in France, where more than 230 people have been killed in assaults in the past two years.
Emergency rule has been in place since Islamist militants killed 130 people in Paris in November 2015, giving police wider search and arrest powers to target suspects considered a threat to security.