More than 750kg (1,654lb) of cocaine - most of it 83% pure - has washed up in plastic packages along France's Atlantic coast this month.
The drugs have been found on beaches from the Landes, in the far south, to the Loire estuary west of Nantes.
Prosecutors in Brittany are collecting the seized bundles. They do not yet know where the drugs came from.
In Gironde, near Bordeaux, several beaches have been closed. People have been told not to move any of the drugs.
The haul's street value so far is estimated to be as much as €60m (£51.5m; $66m).
"Any discovery must be reported immediately to the police or gendarmes [paramilitary police] without touching [the drugs]," the prosecutors' office in Rennes, Brittany's capital, said.
Transporting drugs such as cocaine can land an individual with a 10-year jail term in France.
Rennes prosecutor Philippe Astruc said the 763kg of cocaine found on beaches "probably comes from South America, and it is worth very large sums of money".
Police also warned that the purity of the cocaine made it a health risk to anyone coming into contact with it.
The prosecutors are liaising with the US Drug Enforcement Administration and European anti-narcotics police in their investigation.