A replica of an ancient arch destroyed by ISIS in Syria has been unveiled in London's Trafalgar Square today.

The Arch of Triumph in Palmyra was targeted by jihadists when they overran the city – a UNESCO world heritage site – in May last year. Militants blew up a number of important historic and religious monuments, including the Temple of Bel, which was originally consecrated to a Mesopotamian god but later served as a church and a mosque.

Palmyra was recaptured by Syrian government forces in March, and officials have pledged to rebuild the city.

The erection of the arch in London today was watched by hundreds of onlookers, who mayor Boris Johnson said were gathered "in defiance of the barbarians" in the Middle East.

"Daesh [ISIS] and other terrorist organisations seek to destroy democracy and obliterate history through the appalling acts of terrorism and murder they commit around the world," he said.

"I'm very proud that the Institute of Digital Archaeology is bringing the first monumental scale reconstruction of Palmyra's Triumphal Arch to London in a stand against this violence and in the quest to share the experience of this irreplaceable artefact with as many people as possible. Antiquities like this belong to all mankind and it is imperative that we all strive to safeguard our common heritage."

Maamoun Abdelkarim, Syria's director of antiquities, said the replica, made using 3D technology, was an "action of solidarity".

"The life of the Syrian people rests on their cultural identity, and Palmyra represents one of the most unique and exceptional cultural heritage sites, not just in Syria but the whole world," he added.

"We know that the plans to restore Palmyra to its former glory are grand, but they can be realised if the task is treated as a global mission."

The arch is on display as part of World Heritage Week, and will be moved around the world after three days in London.