Hundreds of litres of toxic wastewater have leaked into Paris' Seine River from a plant belonging to Franco-Swiss cement giant Lafarge-Holcim, local reports said, sparking outrage from French officials.
Lafarge acknowledged the spillage but insisted it had been the result of sabotage, rather than an intentional act by the company.
The contaminated liquid is composed of a mixture of cement, wastewater treatment liquid and plastic microfibres and comes from a Lafarge site in the east of the city, Europe 1 radio said.
"It's a complete environmental scandal, as we have been working with our partners for a long time to improve the quality of the River Seine," Paris Mayor Anne Hidalgo said on Twitter.
The city government "will ask prosecutors to open a case for these grave acts that harm our environment," she added.
Paris prosecutors told AFP they had opened an investigation into pollution of the river with harmful products on August 27.
In a statement, Lafarge said it was the "victim" and said the spillage was provoked by a "malicious act". The cement giant said the leakage has now been stopped and controls reinforced.
Deputy mayor Emmanuel Gregoire said the town hall was "very angry" with the firm, which should have informed the city government of the problem.
This incident "will not be without consequences on our overall attitude on these kinds of sites," Gregoire added.
Since Tuesday, city-dwellers have been informing Paris authorities of harmful practices of a range of cement-making companies, such as toxic waste outpourings into the river, the official said.
Sanctions are not heavy or dissuasive enough, he added.
Environmental activists have previously accused Lafarge of contaminating the Seine and in February staged demonstrations against the Paris site.
Environment Minister Barbara Pompili said on Twitter that a team from her ministry would be going to the scene and those responsible would face justice.
In April 2019, France's Vinci firm was accused of discharging cement wastewater in the River Seine. The company was subsequently fined 50,000 euros ($60,000).