Local officials and media said 96 people died in the Serrana mountain region north of Rio Tuesday and Wednesday after extremely heavy tropical rain sent hillsides sliding into towns and rivers broke their banks.
One town, Teresopolis, counted 71 deaths, according to a tally given to French news agency by the mayor's office.
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Nearby Petropolis suffered at least 18 deaths, a municipal official said.
Another seven deaths occurred in Nova Friburgo, another town in the region, local media reported.
At least three firemen died when they were engulfed by a wall of mud as they were carrying out a rescue.
The town's power was cut off and the telephone system had collapsed.
The deaths added to 13 counted in Sao Paulo Monday and Tuesday, bringing the overall death toll for southeast Brazil to 109 so far this week.
Teresopolis's mayor, Jorge Mario, told the GloboNews television station: "It's a huge catastrophe, a major disaster."
Images from helicopters showed torrents of water cascading down mountains over his town of 180,000 people, and houses swamped in tons of mud.
Mario said around 500 people had been left homeless and dozens of bridges and roads had been destroyed.
He warned the death toll could rise even further as emergency crews made it through to remote stricken areas.
A local civil defense chief, Colonel Flavo Castro, called on locals "to take refuge in safe places: churches, schools."
He said a helicopter was being used to evacuate people cut off.
Heavy rains common during Brazil's summer wet season were intensified this week by a cold front which doubled the usual precipitation.