Fast-rising water levels in the state of Rio Grande do Sul were straining dams and particularly threatening economically important Porto Alegre, a city of 1.4 million.
The Guaiba River, which flows through the city, is at a historic high of 5.04 metres (16.5 feet), well above the 4.76 metres that had stood as a record since devastating 1941 floods.
Authorities scrambled to evacuate swamped neighbourhoods as rescuers used four-wheel-drive vehicles, and even jet skis, to manoeuvre through waist-deep water in search of the stranded.
In addition to the 69,200 residents forced from their homes, civil defence also said more than a million people lacked access to potable water amid the flooding, describing damage as incalculable.
Rio Grande do Sul Governor Eduardo Leite said his state – normally one of Brazil’s most prosperous – would need a “Marshall Plan” of heavy investment to rebuild after the catastrophe.
In many places, long queues formed as people tried to board buses, although bus services to and from the city centre were cancelled.
The Porto Alegre international airport suspended all flights on Friday for an undetermined period.
President Luiz Inacio Lula da Silva posted a video of a helicopter depositing a soldier atop a house, where he used a brick to pound a hole in the roof and rescue a baby wrapped in a blanket.
G7 urges China and others to help climate finance: ‘join us’
G7 urges China and others to help climate finance: ‘join us’
Joao Guilherme, a 23-year-old salesman, found his way to safety in the state capital – but without his mobile phone.
“I have no communication with anybody, I’m very shaken,” he said.
The speed of the rising waters unnerved Greta Bittencourt, 32, a professional poker player.
“It’s terrifying because we saw the water rise in an absurd way, it rose at a very high speed,” Bittencourt said.
With waters starting to overtop a dyke along another local river, the Gravatai, Mayor Sebastiao Malo issued a stern warning on social media platform X, saying, “Communities must leave!”
He urged people to ration water, after four of the city’s six treatment plants had to be closed.
In a live transmission on Instagram, Governor Leite said the situation was “absolutely unprecedented”, the worst in the history of the state, home to agro-industrial production of soy, rice, wheat and corn.
Residential areas were underwater as far as the eye could see, with roads destroyed and bridges swept away by powerful currents.
Rescuers faced a colossal task, with entire towns inaccessible.
At least 300 municipalities have suffered storm damage in Rio Grande do Sul since Monday, according to local officials.
Roughly a third of the displaced have been brought to shelters set up in sports centres, schools and other facilities.
The rains also affected the southern state of Santa Catarina, where one man died on Friday when his car was swept away by raging floodwaters in the municipality of Ipira.
The president, who visited the region on Thursday, blamed the disaster on climate change.
02:17
Dozens killed in highway collapse in China’s rain-hit Guangdong province
Dozens killed in highway collapse in China’s rain-hit Guangdong province
The devastating storms were the result of a “disastrous cocktail” of global warming and the El Nino weather phenomenon, climatologist Francisco Eliseu Aquino told Agence France-Presse on Friday.
South America’s largest country has recently experienced a string of extreme weather events, including a cyclone in September that claimed at least 31 lives.
Aquino said the region’s geography meant it was often confronted by the effects of tropical and polar air masses colliding, but these events have “intensified due to climate change”.