The company blamed a “malicious act” by someone who deliberately damaged a protective panel, allowing waste water from plant activities to leak into the Seine, which flows through the French capital.
The pollutants were not identified.
The Paris prosecutor's office said Wednesday an investigation has been opened into the dumping of dangerous substances. The regional health service and Paris police said administrative investigations are also under way, and that the dumped substances have not affected the city's drinking water supply.
LafargeHolcim, based in Switzerland, said it has filed a legal complaint over the incident and has taken “all necessary measures" to stop the flow and repair the damage. It promised to tighten security at all its plants.
The cement factory, in southeast Paris near the Accor Arena at Bercy that houses sports tournaments and major concerts, last underwent an environmental inspection in February 2019, and no violations were found, the health service said.
Swimming and fishing in the Seine are banned in Paris.