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.
Water levels in Magdeburg stood at 7.44 m on Sunday, nearly four times higher than normal
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.
People in Hungary have been warned to prepare for their country’s worst floods ever as the Danube is set to reach record levels this weekend.
“We are facing the worst floods of all time,” said Prime Minister Viktor Orban.
Europe’s second longest river is set to hit unprecedented levels in the capital Budapest in the next few days.
A state of emergency has been declared, and thousands of volunteers worked overnight to reinforce the banks of the swelling river.
Water levels are set to reach reach 8.85 m (29 ft), some 25 cm (10 in) higher than the Danube’s previous record high in 2006.
Emergency workers have set up camps along the river as residents packed sandbags around their homes amid an atmosphere of concerned expectation.
Kristalina Georgieva, the EU Commissioner for International Cooperation, Humanitarian Aid and Crisis Response, tweeted: “Hungary well prepared for highest ever measured water levels on Danube. We are monitoring & ready to assist.”
Budapest Danube is set to reach record levels this weekend
Viktor Orban, who spent the night at a military barracks in the flooded western city of Gyor, said recent dry weather in Austria and Germany, as well as a hot forecast for Hungary over the weekend, gave reason to hope that Europe’s worst river floods for more than a decade could soon be over.
The Danube peaked on Thursday in the Slovak capital Bratislava, where the main flood defenses held firm.
In northern Germany, workers piled sandbags along the banks of the River Elbe as waters rose, after widespread flooding further south.
As flood waters receded to the south and east, defense work continued apace near Lueneburg in Lower Saxony.
Tens of thousands of people have been evacuated from at-risk areas in Germany, where the flooding is worse than that recorded in 2002.
On Thursday the Elbe flooded parts of Dresden as it peaked nearly 7 m (22 feet) above its normal level, but the city’s historic centre remained unscathed.
Upstream along the Elbe in the Czech Republic, emergency workers used boats to shuttle supplies to stranded people as large areas remained under water.
Widespread flooding in central Europe has inundated swathes of Austria, Germany and the Czech Republic, killing at least 15 people.