Hungary has declared a state of emergency in three western counties after sludge from an alumina plant killed three people and injured 120.
Red chemical sludge flooded out of a burst dam, affecting at least seven villages and towns including Devecser, where it stood 2m (6.5ft) deep.
The sludge is said to contain caustic material and to be toxic if ingested.
While the cause of the deaths was unclear, most of the people injured had suffered chemical burns.
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Some 600,000-700,000 cubic metres (21m-24m cubic feet) of sludge escaped from the plant, 160km (100 miles) from the capital, Budapest.
The state of emergency was declared in Veszprem, Gyor-Moson-Sopron and Vas.
Troops in protective gear have been helping to clean up the spill, while plaster is reportedly being poured into the Marcal river to bind the sludge and stop further flooding.
Tamas Toldi, mayor of Devecser, told MTI news agency that between 80 and 90 people had been taken to hospital with chemical burns. Six people were reported missing.
Burns caused by the sludge could take days to reveal themselves and what might seem like superficial injuries could later cause damage to deeper tissue, Peter Jakabos, a doctor on duty at a hospital in Gyor where several of the injured were taken, said on state TV.
There has been no official explanation of the cause of the disaster, but weeks of heavy rain are likely to have played a role, the BBC’s Nick Thorpe reports from Budapest.
The sludge escaped from a reservoir at the Ajkai Timfoldgyar plant in the town of Ajka. Police say they have confiscated documents from the company’s headquarters.
The sludge entered Devecser and Kolontar, where several hundred houses were flooded and dozens of people were evacuated. Five other localities were under threat.
Rescue workers used an axe to cut through the living room door of Tunde Erdelyi’s house in Devecser, to let the red liquid flow out, the Associated Press news agency reports.
“When I heard the rumble of the flood, all the time I had was to jump out the window and run to higher ground,” she said.
Robert Kis, her husband, said his uncle had been taken to Budapest by helicopter after the sludge “burnt him to the bone”.
Hungary’s disaster agency said 390 residents had to be temporarily relocated and 110 were rescued from flooded localities.
Zoltan Illes, state secretary for the environment ministry, said after visiting Kolontar on Tuesday that it was the worst chemical accident in Hungary’s history.
There were fears, he added, that the sludge could reach the Raba and Danube rivers.
Alumina, a synthetically produced aluminium oxide, is a white or nearly colourless crystalline substance that is used as a starting material for the smelting of aluminium metal.
It also serves as the raw material for a broad range of advanced ceramic products and as an active agent in chemical processing.