LONDON — A landmark Roman arch that was destroyed by Islamic State fighters in Palmyra, Syria, stood proudly once again on Tuesday, this time as a replica built from digital models that was installed in Trafalgar Square in London.

To create the roughly 20-foot-tall marble replica, which weighs around 11 tons, researchers built a 3-D computer model of the arch by compiling dozens of photographs taken by archaeologists and tourists in Palmyra before the Islamic State moved in. Robots in Italy then used the computer model to carve the marble replica.

Mayor Boris Johnson of London unveiled the piece on Tuesday, as crowds gathered in the square and on the steps leading to the National Gallery.