Hundreds of homes in Serbia, Bulgaria and Romania have been flooded as the Danube, swollen by heavy rain and snow melt, rises to record levels.
The river has swamped parts of the Bulgarian town of Lom
A state of emergency has been declared in some areas as the authorities try to shore up defences and move out people threatened by the rising water.
In Romania, authorities carried out controlled flooding to divert water away from populated areas.
Last summer floods killed dozens and damaged farmland and infrastructure.
"We must not relive the nightmare of last year," Romanian Interior Minister Vasile Blaga said.
"We must act quickly to prevent the loss of human lives."
Power plant threat
The Danube is expected to rise as flood waters from central Europe make their way south.
Romanian Prime Minister Calin Tariceanu told a news conference that in the next two days they expected the Danube to pass its highest level for a century.
A state of alert has been declared in 12 areas bordering the river, the French news agency AFP reported.
On Thursday several hundred people were removed from areas bordering the river as 300 houses in the south were flooded, according to the agency.
About 500 police were reportedly working to shore up dykes in areas along the Danube.
- One of the Balkan's biggest power plants, on the Danube in the east, could be partly shut down if the river rises above the dam, state television reported
- Streets in lower sections of the capital were submerged
- In the north-western town of Vidin the Danube flooded a park around the medieval Vida fortress, the Associated Press reported
- In the town of Lom, the river submerged the port and hundreds of homes in surrounding villages, according to the agency.