During a press conference on Wednesday, state governor Eduardo Leite said he had spoken with President Luiz Inacio Lula da Silva seeking all possible federal assistance.
"We are experiencing in Rio Grande do Sul the worst moment, the worst disaster in our history. It is absolutely, absurdly, extraordinarily serious what is happening in Rio Grande do Sul right now," Leite said.
"And unfortunately, it will get worse."
Lula is due to travel to the state on Thursday.
According to the governor, storms have caused the most extensive devastation in the state in recent years, leaving several towns isolated after bridges collapsed and roads were destroyed amid floods and mudslides.
Leite declared the suspension of classes statewide for the remainder of the week and emphasised that he had requested full support from the armed forces "to effectively participate in coordinating this moment, which is akin to facing a war".
Authorities tallied more than 3400 displaced people in the wake of storms that led to rising river levels and flooding in various parts of the state, affecting 114 municipalities.
In a Tuesday report, the state's government predicted a flood risk "across practically the entire state" given the prospect of continued "heavy precipitation" in the coming days.
The bulletin also said flooding would be severe in the basins of many rivers, with the possibility of flash floods and mass movements of earth in regions with steep slopes.
