During the celebrations, one of the most important events in the 1.2 billion-member Roman Catholic Church, faithful make pilgrimages to Rome and other religious sites around the world. This will be only the 29th Holy Year in the Church since the tradition started more than 700 years ago.
Holy Years normally take place every 25 years unless a pope decrees an extraordinary one to bring attention to a particular need or topic, such as this one on the theme of mercy. The next Holy Year had been scheduled for 2025 before Francis decided to call a special one.
Security had always been expected to be tight for the opening ceremony, which is estimated to draw up to 100,000 people in St. Peter's Square, but it was ratcheted up after the attacks by Islamist militants in Paris and California.

"The risk of terrorism will never be zero, we cannot provide absolute security, but we are working to attain the level of security that people rightly ask for," Rome police chief Franco Gabrielli said.
The airspace over most of Rome will be closed for the day and city authorities have banned the transport of petrol, gas, weapons, explosives and fireworks.

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