Reuters_ Graffiti artists have scaled and tagged the soaring stone entrance to Milan's Galleria Vittorio Emanuele II, the prestigious shopping arcade at the heart of the city.
Video posted online showed three figures in black spraying a green inscription as they edged around an upper ledge of the four-storey edifice, watched by tourists below in the square that faces Milan's cathedral.
Local police said the group were spotted around 10:20 p.m. (2020 GMT) on Monday night but disappeared before officers could catch them.
An investigation was underway to try to identify the group and assess how they could have accessed the top of the facade, which is meant to be out of bounds, police added.
The 19th century Galleria is home to shops, restaurants and hotels that could have been used to gain access to the site.
The tagging appeared to be the work of graffiti writers rather than politically-motivated groups or the environmental protesters who have targeted Italian museums and architectural treasures in recent months.
The stunt angered deputy Prime Minister Matteo Salvini who said the perpetrators should be "taught a lesson they will never forget," with prison sentences, fines or community service.