One smashed the front door with a sledgehammer, and the others ran inside with machine guns and pistols, the statement said. Speaking French, a group of men ordered the guests and employees to the floor, while their accomplices emptied registers in the basement and an upper floor, it said.
They couldn't get into the strong room, despite shooting at the locked door, prosecutors said.
"The criminals fired a number of shots, but luckily no one was hit," said the statement.
It said some guests and one security guard were lightly injured from being hit or stepped on, and that Swiss and French police were trying to track down the men.
Swiss police said the men fled the casino quickly, driving away in two silver Audis with French license plates.
A woman who accidentally drove between the two getaway cars and blocked the casino's exit was pulled from her vehicle and beaten, police said. They said it was possible that the men fired a shot at another car during their high-speed escape.
The Grand Casino is located little more than 200 meters from Switzerland's border with France.
The men made off with "several hundreds of thousands" of francs (dollars), police said.
The incident recalls a brazen raid March 6 on a poker tournament at a downtown hotel in Berlin. In that heist, attackers armed with a revolver and a machete made off with euro240,000 ($320,000) in jackpot money.
German authorities say they have arrested five suspects.