People climb along a fence to pass a flooded area in Rio de Janeiro. — AFP Photo

RIO DE JANEIRO — Torrential rain left at least 10 people dead in Rio de Janeiro, officials said on Tuesday, as emergency workers rescued people trapped by the downpour and clean-up efforts gathered pace.

The unusually heavy rain began Monday evening and continued into Tuesday, triggering widespread flash flooding that turned some streets into raging rivers, toppled trees and swept away cars as the state emergency agency declared a crisis.

The southern zone of the city, which includes the tourist hot-spots Copacabana and Ipanema as well as several impoverished favela areas, has been hardest hit.

Heavy runoff from neighbouring hills gushed through some apartment buildings and shops, leaving behind mud and debris.

The bodies of three people were found in a car buried behind an upmarket shopping centre on Tuesday afternoon, while two women died when a house in a favela collapsed, the Fire Department said in a statement.

The other victims included a man who died after he was electrocuted in a house and another who was caught in a torrent of water.

The ferocity of the storm stunned some residents, who are long used to wild weather battering their city.

"I've never seen or witnessed anything like this. I've seen it happen elsewhere but never here," Guillerme de Oliveira, 59, said.

"I'm really amazed at all of this."

Cars and public buses were crushed by fallen trees, and sections of streets were ripped up by the force of the water.

Many people returning home Monday evening or trying to go to work on Tuesday were drenched as they waded through knee-deep water in the relentless downpour.

Some people climbed fences to avoid flooded streets, while others tied plastic bags around their feet to protect themselves from the polluted water.

Beaches normally packed with people sunbathing or playing sports were empty as brown water flowed into the ocean.

"Many houses in Rio are built in areas that are inappropriate, in places where water flows. It's a tragedy and we have tried to warn these people," Rio de Janeiro mayor Marcelo Crivella told local media.

Schools throughout the city suspended classes and power was knocked out in some areas as heavy rain complicated rescue and clean-up efforts.

Crivella said the downpour had been "totally atypical."

Rio de Janeiro state governor Wilson Witzel expressed sympathy for the "families of the victims" on Twitter.

"The whole state is helping in what is necessary," he said.

Heavy rain often causes flash flooding in Rio de Janeiro as the volume of water overwhelms the city's creaky infrastructure and brings traffic to a grinding halt.

City government spending on projects including drainage works fell more than 79 percent from 2016 to 2018, economic news site Valor reported.

Torrential rain in February killed at least six people in Rio de Janeiro. — AFP