With damages estimated at more than $300mn, the flooding was declared a national disaster by Viktor Yushchenko, the Ukrainian president.
Yushchenko had to leave festivities marking the 1,020th anniversary of the country's adoption of Christianity and flew to the heaviest-hit Ivano-Frankivsk region.
Oleksandr Turchinov, the first deputy prime minister, said: "Ukraine has not seen anything like this in 100 years".
Authorities in Bucharest, the Romanian capital, said a 30-year-old mother, her son and another youngster, who had taken refuge from the swollen waters in a house built on a hill, died when a landslip swept the building away in the northern district of the country.
Victor Dobre, the secretary of state of the interior ministry said that two other people had also been swept away by the floodwaters
At least 8,200 people had been evacuated and 8,600 homes flooded in Bucharest, he said, while more than 3,000 security forces and volunteers had been mobilised to build dykes, help evacuate people and animals in danger and to distribute aid.
Storms will continue for at least another 24 hours and the water level is expected to rise, a meteorological service official said.