Heavy rains late Tuesday and Wednesday (local time) paralysed much of Rio de Janeiro, a tropical metropolis scrambling to improve infrastructure to prepare to host the 2014 World Cup and the 2016 Olympics.
More rain fell overnight around the city, Brazil's second biggest, than would normally be expected during the entire month, meteorologists said.
The downpour flooded major thoroughfares, toppled houses in working class suburbs, disrupted train and flight schedules and created such chaos that Mayor Eduardo Paes asked residents to stay home.
Thousands of people in the Baixada Fluminense, a string of working-class suburbs that extends inland from central Rio, sought emergency shelter because of flooded homes.
In some of the neighbourhoods, residents protested what they considered a slow response by authorities. Along at least one major highway, looters ransacked trucks and other vehicles that were stranded by the water.
To help ensure order, President Dilma Rousseff agreed to send federal security personnel as backups for state police. In a statement, Rio's state government said the governor had also asked the federal government to help provide mattresses, potable water and emergency food supplies.
No deaths had been reported but some people had been injured.
Though December is part of the annual rainy season, the intensity of the rains underscored longstanding concerns about flooding in a coastal city where slipshod development and poor public oversight in past decades led to an urban sprawl across a floodplain between nearby mountains and the sea.
''We are now dealing with the problems of opportunistic development that puts people and property at risk,'' said Moacyr Duarte, an engineer and researcher on disaster management who also advises the city on preparedness.
''The authorities never should have let much of this be built.''
Television footage and pictures sent to local media by harried residents showed passengers standing atop buses half submerged in muddy water.
One Rio resident was photographed riding a jet ski past a trapped bus, while some motorists were stuck in cars that risked being swept away by rising runoff.
The flooding came just six months before Rio, along with 11 other Brazilian cities, welcomes hundreds of thousands of visitors for World Cup soccer games.
Two years later the city will host the summer Olympics, an event for which it is building new venues, bus and rail lines and other facilities.
While local officials pointed to advances since past flooding, when smaller volumes of rain led to deaths, they were concerned about long-term climate forecasts that suggest downpours will intensify in southeast Brazil in coming years.
Long accustomed to hosting throngs of tourists and major events, such as annual Carnival celebrations and a big seaside New Year's party, the city and surrounding region, an area of more than 10 million people, have repeatedly been caught off guard by the weather.
Last July, when rains turned a rural field they had prepared for a campsite and outdoor sanctuary into a bog during a visit by Pope Francis, authorities had to relocate his final mass.
In 2011, heavy rains in the mountains outside Rio caused major mudslides and killed more than 900 people.
Forecasts predict rain in Rio at least through the end of the workweek.