Petropolis received more than 10 inches of rain (29 centimeters) in three hours on Tuesday, destroying some 80 homes and grinding life to a halt. Officials on Thursday said more than 110 people were still missing.
The hilly colonial town, just north of the city of Rio de Janeiro, is a popular tourist destination.
Brazilian President Jair Bolsonaro, who departed for Russia Monday, told reporters from Moscow that his administration intends to "offer the mayor what we can."
He said he would release federal funds to help "restore traffic in the region." Bolsonaro tweeted about the tragedy on Tuesday as well, offering prayers for those impacted by the rains.
Officials shocked by damage
"The situation is almost like war ... cars hanging from poles, cars overturned, lots of mud and water still," Rio de Janeiro Governor Claudio Castro told reporters from the affected region.
Petropolis' city hall declared three days of mourning for victims as more than 700 people had to leave their homes and were moved to schools and temporary shelters.
The fire department and local civil defense teams have been working round the clock to search for survivors.
"Working overnight, with poor lighting and soggy ground, is always a challenge. But I'm determined to work tirelessly in our search for survivors," Rio de Janeiro's civil defense head Leandro Monteiro said.
More than 500 rescue workers, alongside neighbors and victims' relatives, are still searching for survivors according to emergency services. Though 24 people were rescued from the mud and rubble in the first hours after the disaster, hope of finding any more alive is growing slim.
Authorities say that at least 35 more people remain unaccounted for and that at least 13 children are among the dead.
As rescue efforts continue, workers at the local morgue have struggled to identify victims among the growing number of fatalities. They have also been forced to use a refrigerated truck to store bodies as the morgue itself has reached capacity.
Assessing the extent of disaster
The rainfall was the heaviest registered in Petropolis since 1932. Weather forecasts predict more rain for the area overnight. Power has not yet been restored.
Social media footage of the town showed badly damaged roads and buried homes.
Residents told news reporters they were distraught at the extent of damage as torrential downpours swept away their homes.
"I've been living here for 44 years and never saw anything like that ... All my friends are gone, they are all dead, all buried," said local resident Maria Jose Dante de Araujo.
Deadly floods and heavy downpour, a recurrence in northeastern Brazil and Sao Paulo state since December, have threatened to delay harvests in the region and have shut down mining operations in the state of Minas Gerais, just north of Rio.