By Catarina Demony

LISBON One of three huge billboards erected by activists to raise awareness of sexual abuse by clergy has been removed following the arrival in Portugal of Pope Francis, campaign group This Is Our Memorial said on Wednesday.

Francis arrived in Lisbon to attend the World Youth Day event, an event devised by the late Pope John Paul II for Catholics in their teens or early 20s and held every two or three years in a different city.

The event comes less than six months after a report by a Portuguese commission said at least 4,815 minors were sexually abused by clergy - mostly priests - over seven decades. The commission in charge said that was just the "tip of the iceberg".

One of the three billboards was put up on the Almirante Reis avenue, one of Lisbon's longest and busiest streets, and reads: "4,800+ children abused by the Catholic Church in Portugal". It had 4,815 dots representing each victim.

Another billboard, in the municipality of Oeiras, where events related to the World Youth Day will also take place, was removed, according to the group that organised the awareness campaign, called This Is Our Memorial, and images shared on social media.

"In the municipality of Oeiras all illegal advertising is removed - in this and in all cases," the municipality was quoted by Publico newspaper as saying.

The municipality did not immediately reply to a Reuters request for comment.

Questioned about it at a daily World Youth Day briefing by security services, spokesperson Artur Querido said the police had no information to share for the time being.

The group described the removal of the billboard as "censorship".

The Church had promised a memorial would be unveiled during the week-long event, but a date has not been set, with the Church saying the project was still being studied.

Francis, who will be in Portugal until Aug. 6, is expected to meet privately with abuse victims.

Lisbon Patriarch Manuel Clemente said on Monday that the Portuguese church's commitment to tackle clergy sexual abuse was "total".

"Nothing can repair the experience and lives of these more than 4,800 victims," This Is Our Memorial said on its website. "What we can and should do is remember them. Give them a voice. So that what happened never happens again."

(Reporting by Catarina Demony; Editing by David Latona, Alison Williams and Conor Humphries)