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.
Bulgaria is set to vote in a referendum on whether a new nuclear power plant should be built.
The opposition Socialist party called the vote because it wants the government to reverse its decision not to build a new plant at Belene.
The first referendum in Bulgaria’s post-Communist history has polarized opinion and is seen as a precursor of general elections later this year.
The referendum would only be valid if at least 60% of the electorate votes.
The government says it supports the provision of nuclear power from an existing plant at Kozloduy, but that it does not have the 10 billion euros it says would be needed to build a new plant.
Prime Minister Boyko Borisov told local media that this would remain the case even if Bulgarians voted in favor of a new nuclear plant.
Bulgaria votes in Belene nuclear power plant referendum
Bulgaria had to close four of its old reactors at Kozloduy as a precondition for its 2007 EU membership.
The government froze plans to finish the plant at Belene last year, when work at the site on the southern bank of the River Danube was already well under way.
The Socialists are seen as closely linked to the Belene project, having granted a construction contract for the plant to Russian state company Atomstroyexport in 2008.
They say Belene would now cost 4-6 billion euros to complete, and would lower electricity costs for consumers.
Environmentalists had opposed the plant, which had first been proposed when Bulgaria was under communist rule.
A rapid thaw caused a real chaos on the River Danube in the Serbian capital Belgrade, where ice damaged boats, pontoons and floating restaurants.
The thick ice covered one of Europe’s busiest waterways during the recent freeze, but began to break up on Sunday as temperatures rose.
In Belgrade, boats crashed into each other but there were no immediate reports of casualties.
One boat owner said the ice had moved so fast, boats could not be saved.
“The damage will be hundreds of thousands of Euros for sure,” Dragan Jovanovic was quoted as saying by Reuters news agency.
Another boat owner, Mihailo Svilaric, told Reuters only a “handful” of boats remained intact out of about 100 moored in the Serbian capital’s Kapetanija marina.
A rapid thaw caused a real chaos on the River Danube in the Serbian capital Belgrade
Debris was scattered among the breaking ice for hundreds of metres along the river, and several floating restaurants, barges and boats were beached on river banks after the ice snapped anchor lines.
Local media reported that Belgrade emergency services said there was no ice risk to bridges and other infrastructure in the city, and there was no threat of flooding.
The Danube flows 2,860km (1,777 miles) through nine countries and is vital for transport, power and industry.
It has been almost entirely frozen from Austria to the Black Sea, in Romania, the Associated Press news agency reports.
Ice more than 30cm (11in) thick in places broke up over the weekend as temperatures rose.
Belgrade river officials advised boat owners to constantly monitor their property but not to try to recover it from the water as of Monday morning.
Belgrade resident Miroslav Jagas told AP he believed that complacency had aggravated the problems caused by the freezing weather.
“We have not seen weather like this in a long time,” he said.
“People were relaxed, the boats stayed there, the icebreakers did not remove the ice on time.”
At least 20 people have died from the cold in Serbia in recent weeks and economists say damage from the cold snap may cost Serbia as much as 500 million Euros ($660 million).
Some 3,300 people remain stranded by snow and ice in rural areas, where they can only be reached by helicopter.