At least 10 tonnes of dead fish have been pulled out of the River Oder which flows along part of Poland's border with Germany - as officials warn people not to enter the water due to possible contamination.
Anglers and volunteers removed the dead fish from the 200km (124 mile) stretch of the river north of Olawa in southwest Poland, the head of the agency which manages the country's national waters has said.
Przemyslaw Daca, head of State Water Holding, also called the situation a gigantic ecological catastrophe.
Poland's prime minister Mateusz Morawiecki has vowed to punish those responsible after environmental authorities said they notified prosecutors about potential contamination of the country's second longest river.
Ewa Drewniak, a biologist working with opposition political group Civic Coalition, has accused the government of not responding quickly enough.
"Dead fish have been flowing in the Oder for the past two weeks and people have not been informed about it, I've seen scores of people bathing in the river a week and a half ago, they were not aware of the danger, this is scandalous," she said.
Regional environmental protection authorities in the Polish city of Wroclaw said Oder water samples taken on 28 July showed an 80% probability that they contained mesitylene, a toxic substance, although this was not present in samples taken after 1 August.
Low river levels due to the drought in Europe might have aggravated possible contamination, Mr Daca said on Wednesday.
He added they suspected a strong oxidizing agent might have gotten into the water causing oxygen levels to spike, which can harm fish.
Mr Daca also said there was no reason for panic and the situation was improving.
Several German districts bordering Poland warned locals to avoid the river water and not to eat fish from the Oder as long as the cause of the fish deaths was unclear.
"The reports from the Oder are simply terrible," Michael Kellner, a senior politician of Germany's The Greens party, said on Twitter.