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 7.44 m (24 ft) on Sunday, nearly four times higher than normal (2 m).
In Hungary, water levels on the Danube are expected to peak on Sunday after the worst floods in more than a decade.
At least 15 people have died in the floods in Central Europe, which will cost billions of euros to clean up.
The authorities in Germany are investigating an anonymous letter threatening attacks on several dams.
The motive behind the threats is not known, but the threat is being taken seriously.
In the Hungarian capital Budapest, flood defenses have been strengthened and appear to have held up.
Along more than 700 km (470 miles) of the River Danube, thousands of people, including many volunteers and even convicts from the prisons, worked to reinforce earth and sandbag barriers,
Late on Friday, residents were moved out of their homes in the village of Gyorujfalu, near the city of Gyor, because a dyke weakened after a landslide.
No flood-related deaths have so far been reported in Hungary.
The river is forecast to peak at around 9 m in Budapest late on Sunday. Flood barriers now stand at 9.50 m.
In Magdeburg, more than 23,000 people left their homes as flooding increased and power was shut off.
Flood defenses on the Elbe and the River Saale have been weakened by days of heavy rain.
Another dam is in danger, reports the regional broadcaster MDR, and more evacuations may have to take place.
German President Joachim Gauck visited the flood-stricken city of Halle, near Magdeburg, and paid tribute to the way its inhabitants had coped.
“It is fantastic to see how different people get together to help others. And you notice that many of them are exhausted, but they are happy and it seems to motivate them,” Joachim Gauck said.
The flood crest is expected to reach northern Germany in the coming days.