Wielding baseball bats and pickaxes, between six and ten men burst into the Franck Muller shop on the northern Italian city's famous Via della Spiga and smashed the glass cabinets, grabbing watches and jewellery before fleeing on foot, Italian media reported.
The shop's manager began chasing after the men, who were dressed in black and were wearing ski masks which covered their faces, but was thwarted when they threw three Molotov cocktails behind them, sparking panic and wounding two other shopkeepers on the street.
"I saw a hooded man running, he shouted 'Molotov' and then there was an enormous bang," one witness told La Repubblica, while another said: "it was crazier than a film."
A fourth unexploded cocktail was found nearby.
A 39-year-old security guard hit in the chest with a bat by one of the robbers was taken to hospital, while another man was treated for light wounds at the scene, the reports said.
"All robberies that have taken place in the luxury fashion quarter have one thing in common: the culprits have always been uncovered and identified," Interior Minister Angelo Alfano said.
"We are confident that it will the case here too," he said.