At least eight people have been killed across central Europe as floods continue to devastate parts of Germany, Austria, Switzerland and the Czech Republic.
Nine other people remain missing following torrential rains, which put much of central Europe on a state of high alert on Monday.
Al Jazeera's Rory Challands, reporting from Prague, said the waters were expected to reach their peak at about midnight.
"Much of Prague is a UNESCO World Heritage site and if the river breaches its banks it is going to do significant damage to one of the world's historic treasures," our correspondent said.
Waters from three swollen rivers gushed into the old town of Passau in southeast Germany, one of the worst-hit cities, pushing water levels to the highest they have been in 70 years.
Much of the city was inaccessible on foot and the electricity supply was shut down as a precaution. Rescuers were using boats to evacuate residents from flooded parts of the city.
With water from the Danube, Inn and Ilz rivers relentlessly pouring into the city, markers set in 1954, when the city suffered its worst flooding in living memory, have disappeared beneath the rising water.
The German army said it had sent 1,760 soldiers to help local authorities and volunteers reinforce flood defences particularly in the south and east of the country.
|Al Jazeera's Nick Spicer reports on the floods
Chancellor Angela Merkel planned to visit flood-hit areas on Tuesday, spokesman Steffan Seibert said.
How her government responds to the emergency could influence the outcome of a nationwide election in September.
"It's perfectly normal that the leader of the government would go to the region and see what is happening for herself," Seibert said.
Shipping was stopped on parts of the Danube and Rhine rivers in Germany, both important arteries for shipping grains, coals and other commodities, because of the high waters.
Authorities in the Czech Republic worked on Monday to erect further protective metal barriers along the Vltava river, which also flows through the capital Prague.
Volunteers piled up sandbags to keep the swollen river from overwhelming the city's historic centre after floods forced factories to close and drove thousands from their homes.
The last time central Europe saw similar floods was in 2002, when 17 people were killed in the Czech Republic, and damage estimated at $26bn was inflicted.
In Hungary, where the capital Budapest is built on the banks of the Danube, state media quoted Gyorgy Bakondi, head of the National Disaster Authority, as saying that 400 people were working on flood defences.
He said water levels in the river could reach or even exceed the height seen in the record flooding in 2002.