Brazilian officials say rains have caused two dams to reach capacity in Sao Paulo state, threatening 12 soaked cities with more flooding.
More than 100 people have been killed by mudslides and flooding triggered by heavy rains across southeastern Brazil since Jan. 1.
Heaviest hit have been Rio de Janeiro and Sao Paulo states.
An official who oversees the dams in Sao Paulo state told the Agencia Estado news agency that at least 6,000 people need to be evacuated from the 12 cities.
The official said two dams in the state can no longer hold water.
While there is no sign the dams might burst, further rain will flow into already full rivers and further flood towns.
Rains are forecast overnight for Sao Paulo.
The city of Sao Paulo on Tuesday marked the 35th day of heavy rains, which have cut off roads and neighborhoods and claimed several lives.