Authorities in the Brazilian state of São Paulo say at least 36 people have been killed in heavy flooding and landslides, forcing some cities to cancel annual Carnival celebrations.
Video showed neighbourhoods under water, flooded motorways and debris left after houses were swept away.
Rescue teams have been struggling to reach survivors and unblock roads.
More than 600mm (23.6 inches) of rain fell in some areas on Sunday, twice the expected amount for the month.
"Search and rescue teams are not managing to get to several places; it is a chaotic situation," said Felipe Augusto, the mayor of the hard-hit town of São Sebastião.
"We have not yet gauged the scale of the damage. We are trying to rescue the victims."
Dozens were missing in the town and about 50 houses had collapsed and washed away, Mr Augusto added, saying that the situation remained "extremely critical".
The state government reported at least 35 deaths in São Sebastião and the mayor of Ubatuba, some 80km (50 miles) north-east, said a young girl had been killed. Hundreds have been displaced and evacuated.
"Unfortunately, we are going to have many more deaths," a civil defence official told newspaper Folha de São Paulo.
Meanwhile, officials say another 228 people have been left homeless, with 338 more evacuated from coastal regions north of São Paulo.
A 180-day state of calamity was declared in six towns in the state: São Sebastião, Caraguatatuba, Ilhabela, Ubatuba, Guarujá and Bertioga.
State Governor Tarcísio de Freitas said he had released the equivalent of $1.5m (£1.2m) in funding to aid in disaster relief.
Carnival events were cancelled across the northern coastline, which is a popular destination for wealthy tourists looking to avoid huge streetside festivities in the big cities.
The festival usually lasts for five days in the run-up to the Christian festival of Lent and the colourful celebrations are synonymous with Brazil.
Latin America's largest port in Santos was also shut as wind speeds exceeded 55km/h (34mph) and waves rose to over a metre, local media reported.
President Luiz Inácio Lula da Silva, who was spending the carnival weekend in the north-eastern state of Bahia, said he would visit the affected areas on Monday.
In a post on Twitter, he sent his condolences to those who had lost loved ones and promised to bring authorities together to provide healthcare and rescue teams.
"We are going to bring together all levels of government and, with the solidarity of society, treat the wounded, look for the missing, restore highways, power connections and telecommunications in the region," Mr da Silva wrote.
More heavy rain is expected in the area, threatening to make conditions even worse for emergency teams.
Extreme weather events such as the floods are expected to become more common as the impacts of climate change took hold.
Last year, torrential rain in the south-eastern city of Petropolis killed more than 230 people.