Four people have died and at least eight are missing as torrential rains in central Europe caused landslides and took rivers to dangerously high levels, according to BBC.
Emergency operations are under way in Austria, Germany and the Czech Republic to deal with record levels of flooding in some places.
Thousands of homes across the region have been evacuated.
The Czech capital, Prague, is on high alert amid fears that floodwater could swamp its historic centre.
Main roads in many areas of central Europe have been closed and rail services cut. In some areas, electricity has been turned off as a precaution.
In Prague, underground stations were closed and businesses and schools shut as city officials braced themselves to see whether the Vltava River would flood its banks. Animals from the city's zoo were also evacuated.
Czech Prime Minister Petr Necas called a special cabinet session on Sunday to co-ordinate the emergency response, and around 1,000 troops have been mobilised to help erect metal barriers and fill sandbags.
"We will do everything to protect people's lives and health," he said. "Tonight and tomorrow will be critical."
The BBC's Rob Cameron in Prague says the city is hoping that the defences it installed after devastating floods in 2002 will work.
At risk is the 14th Century Charles Bridge and other historic buildings close to the river bank, he says.
In Germany, Bavaria's flood alert service warns that the forecast of continuing heavy rain is likely to worsen the flooding affecting the Danube and the Inn, among other rivers in the area.
The German cities of Passau and Rosenheim have declared a state of emergency.
Authorities in Passau, which lies at the confluence of three rivers in Bavaria, said waters could rise above the record levels of 2002.
Heavy rain
Outside Prague, two people were killed and four reported missing after a house collapsed. The body of a man in his 50s was found close to swollen river waters north-east of Prague and two people are missing after their raft overturned south-west of the capital.
In Austria, the meteorological service said two months worth of rain had fallen in just two days.
A man was found dead near Salzburg after being swept away as he worked to clear a landslip.
Two further people are missing in the Salzburg area, according to Austrian media. A third is missing in Vorarlberg.
The army was called in to help civil authorities in the settlement of Taxenbach, south of Salzburg. Their main task was to clear landslides and make roads passable.
Parts of the Pinzgau region, which includes Taxenbach, have been declared a disaster zone.
Bavaria is not the only German state to be affected; towns and cities in Saxony, Thuringia and Baden-Wuerttemberg are also inundated.
The Munich-based newspaper Sueddeutsche Zeitung reports that the German army is to be deployed in Bavaria, Saxony and Thuringia to support the flood-affected areas.
Chancellor Angela Merkel has discussed the floods in phone-calls with the premiers of Bavaria and Saxony, the paper says.
