At least 80 people have died and hundreds of people are missing in Germany and neighbouring Belgium.
Heavy flooding turned streams and streets into raging torrents, sweeping away cars and causing buildings to collapse.
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Storms across parts of western Europe in recent days caused rivers and reservoirs to burst their banks.
The equivalent of two months of rain has fallen on some areas in the last day or two, according to the French national weather service.
It has resulted in several flash floods overnight as rain-soaked soil failed to absorb any more water.
German Chancellor Angela Merkel called the flooding a "catastrophe" and said she was "grieving those who have lost their lives".
"My thoughts are with you, and you can trust that all forces of our government - federal, regional and community - collectively will do everything under the most difficult conditions to save lives, alleviate dangers and to relieve distress."
Where in Germany has the flooding hit?
The BBC has reported that at least 42 people have now been killed in western Germany, after record rain fall.
Authorities in the western German region of Euskirchen said eight deaths had been reported there in connection with the floods.
Officials added that 18 people had died in Ahrweiler county, south of Euskirchen.
Up to 70 people were reported missing after several houses collapsed overnight in the village of Schuld in the Eifel, a volcanic region of rolling hills and small valleys southwest of Cologne.
Dozens more were trapped on the roofs of their houses awaiting rescue. Authorities used inflatable boats and helicopters, and the German army deployed 200 soldiers to assist in the rescue operation.
"There are people dead, there are people missing, there are many who are still in danger," the governor of Rhineland-Palatinate state, Malu Dreyer, told the regional parliament.
"We have never seen such a disaster. It's really devastating."
German Chancellor Angela Merkel said she was distraught by the news of the floods.
"My sympathy goes to the relatives and of the dead and missing," she said during a trip to Washington.
What is the flooding like in Liege?
Across the border in Belgium, the Vesdre river broke its banks and sent masses of water churning through the streets of Pepinster, close to Liege, its destructive power bringing down some buildings.
"Several homes have collapsed," mayor Philippe Godin told RTBF network. It was unclear whether all inhabitants had been able to escape unhurt.
Major highways were inundated in the south and east of Belgium, and the railway service said all traffic was stopped.
The full extent of the damage across the region was still unclear after many villages were cut off by floodwater and landslides that made roads impassable. Videos posted on social media showed cars floating down streets and houses partly collapsed in some places.
Is the EU going to step in?
Yes, EU Commission President Ursula von der Leyen pledged to help those affected.
"My thoughts are with the families of the victims of the devastating floods in Belgium, Germany, Luxembourg and the Netherlands and those who have lost their homes," she tweeted.
"The EU is ready to help."