TV and social media footage from the town of San Sebastiao showed entire neighbourhoods under water, debris from hillside houses swept away by oozing earth, flooded highways and cars destroyed by fallen trees, among other damage.
At least 23 people died in San Sebastiao, according to Mayor Felipe Augusto. A girl was also killed in the town of Ubatuba, news reports said.
Another 228 people were left homeless and 338 were evacuated in the coastal region north of the city of Sao Paulo, the state government said earlier, as rescue crews raced to help those hit by the storm.
The authorities did not give a figure for how many people were missing or injured.
Sao Paulo state governor Tarcisio de Freitas declared a state of emergency in five towns along the coast after flying over areas devastated by the weather. He freed up the equivalent of $1.5 million for rescue operations.
President Luiz Inacio Lula da Silva said on Twitter that he will visit the area on Monday. He has been on holiday since Friday in the northeast state of Bahia.
San Sebastiao, 200 kilometres (120 miles) north of Sao Paulo and where many people from the city spend the pre-Lenten holiday weekend on the beach, was hard hit, as a record 60 centimetres (nearly a foot) of rain fell in 24 hours, city officials said.
That is more than twice the amount that normally falls in a month.
Carnival events in San Sebastiao and elsewhere were cancelled.
"We have not yet gauged the scale of the damage. We are trying to rescue the victims," said Augusto, calling the situation in the town "extremely critical."
"We are working at nearly 50 residences that collapsed under the force of the water and there are still people buried," he told Globonews.
More than 100 firefighters were working on the scene, with the aid of helicopters.
Soldiers were also taking part in the race to help people affected by the downpour.
Lula said government at all levels would work to "take care of the injured, look for missing people and restore roads, energy and telecommunications."
Amid the loss and destruction, authorities said a boy aged two was rescued from a sea of mud, as was a woman who was giving birth.
Extreme weather events fueled by climate change are taking a heavy toll on Brazil.
Torrential rains last year in the city of Petropolis lead to the death of more than 230 people.
