The heaviest rains since records began almost 120 years ago have hit Serbia and Bosnia, cutting off power and causing more than 200 landslides.
At least 20 people have died in the unfolding disaster this week. Thousands have been evacuated from homes in central and western areas of Serbia and in neighbouring Bosnia.
In Serbia 6,000 people have been forced out of their homes.
Thousands more are still awaiting evacuation as river levels reach a critical mark, notably on the Sava and Morava river, weather services said.
Serbia has never seen such a horrible natural catastrophe
Both countries declared a state of emergency on Thursday.
More than 150,000 households have been left without electricity, the AFP news agency reported.
The Serbian government ordered emergency supplies from neighbouring countries, after two hydro-power plants endangered by the floods were closed down.
"Serbia has never seen such a horrible natural catastrophe," Aleksandar Vucic, prime minister, told a meeting of the emergency staff.
Officials warn more landslides are to be expected when swollen Bosnian rivers start recede to normal levels.
According to the Reuters news agency, both Bosnia and Serbia have requested international help.
A Russian emergency team has flown in to join the rescue efforts.
Weather services said the rain could stop by mid-Saturday.
Since Wednesday, as much rain has fallen as normally in three months, Nenad Drobnjakovic, meteorologist, said.
Poland also saw less severe flooding following the torrential rains across eastern Europe.
In neighbouring Croatia, officials were on high alert amid fears that the river Sava will overflow and flood the area in the east of the country.