A dam has broken on the flood-swollen River Elbe in eastern Germany, forcing thousands of people to leave their homes around the city of Magdeburg.
Water levels in Magdeburg stood at 7.44m on Sunday, nearly four times higher than normal.
A BBC correspondent reports an anonymous letter threatening attacks on several dams is being investigated. The motive behind the threats is not known, but the threat is being taken seriously.
Flood defences on the Elbe and the River Saale have been weakened by days of heavy rain. Another dam is reportedly in danger and more evacuations may have to take place.
President Joachim Gauck visited the flooded city of Halle, near Magdeburg, and paid tribute to the way its inhabitants had coped.
Chancellor Angela Merkel has promised 100 million euros ($US130 million) in aid for flooded areas.
"We'll do everything humanly possible when it comes to reconstruction. Germany is sticking together in an admirable way at the moment and it should stay like that," she said on Sunday.