Three men entered the boutique toward 3:40 pm (1440 GMT) while a driver waited in a car, the Monaco prosecutor's office said.
Three men entered the boutique toward 3:40 pm (1440 GMT) while a driver waited in a car, the Monaco prosecutor's office said, adding that no shots were fired and no one was wounded or taken hostage.
The area -- which is also home to Monte Carlo's famed casino -- was sealed off by the police as soon as the alarm was sounded, they said, adding that at least one man was armed.
Clients in the showroom as well as tourists who found themselves in the zone were confined for part of the afternoon in the Monte Carlo Casino and nearby boutiques.
One of the three attackers was found shortly afterwards carrying a gun and some of the stolen jewels.
Officials have not yet put a value on the loot.
The car they drove away in was later found, having been set on fire.
"Sealing off the area also stopped the escape of the two other individuals and the accomplice who was waiting for them in a vehicle," the prosecutor's office said.
The search was continuing Saturday evening for the other suspects, though the police cordon had been lifted and activity was returning to normal, according to an AFP photographer at the scene.
The lights were still on at the Cartier boutique, however, as investigators tried to piece together the robbery.
Psychological help was being provided to the employees, the prosecutor's office said.
Cartier has already been the target of several robberies on the French Riviera, including another daylight attack in May 2015 in Cannes, just days before the opening of the city's film festival.
A man wearing a mask of an elderly person and carrying a machine gun managed to enter the store toward midday, despite a security hatch and a guard.
He then let in two others whose faces were covered with scarves, while a fourth man stood guard outside.
They got away with jewelry and watches valued at some 17.5 million euros ($18.9 million).