Argentina's government declared three days of national mourning from Wednesday after massive flooding killed 54 people in Buenos Aires and the nearby university city of La Plata.
At least 46 people died as flood waters hit La Plata about 60 kilometers (40 miles) south of the capital, following historic rains that swept through the streets, submerging cars as people cowered on rooftops. Most of the victims were found after a second day of record rainfall hit the area.
"The bodies began to appear as the water subsided," said Governor Daniel Scioli, as officials confirmed the death toll and locals piled up chaotic heaps of soaked mattresses and destroyed refrigerators.
A senior city official, Santiago Martorelli, told local television the floods were a "catastrophe," and said La Plata's schools and government offices had been closed.
"There are people on rooftops, in trees waiting for us to rescue them," Martorelli said, adding that firefighters, civil defense workers, police and soldiers have been deployed to the area to help in rescue operations.
The fatalities in La Plata, a bustling university city of just under one million, followed at least eight others in Buenos Aires on Tuesday, when a storm knocked out power lines and downed trees.
President Cristina Kirchner made a surprise visit to La Plata, surveying the devastation by helicopter.
Kirchner spent much of her childhood in La Plata, and her mother, Ofelia Wilhelm, still lives in the same two-story home where the president grew up.
"Mom has lost her power and gas but she does not want to leave her home because the water is close to the door, and she is afraid it might start raining again," Kirchner told reporters.
A staggering record 40 centimeters (16 inches) of rain fell on La Plata during a two-hour period late Tuesday into Wednesday, officials said, knocking out phone lines and leaving about half the city in the dark.
In Buenos Aires, more than 15 centimeters of rain -- an April record -- fell between late Monday and early Tuesday, the weather service said.
Flood waters reached two meters (6.6 feet) in some places, turning roadways in La Plata into a raging river.
"This has never before happened in La Plata," said Argentine Security Minister Sergio Berni.
In nearby Tolosa, meanwhile, 50 cars and other vehicles were stuck after high water encircled part of the town, cutting off roads and preventing authorities from rescuing stranded motorists and passengers.
"We have no batteries, no power," lamented Vanessa Silletti as she spoke by phone to local radio, stranded in her vehicle with a 10-month-old infant.
"I can nurse my baby, but that's about it. We are stuck here, powerless, unable to move," Silletti said.
Authorities said around 2,500 people were still unable to return home as they wait for flood waters to subside, and have taken refuge in emergency shelters.
Maximiliano Miceli, 34, told AFP he was stunned by the flooding in La Plata, where he has never seen heavy rain, nor such devastation.
"This is the first time that this has ever happened," he said, as he wiped away muddy water from inside his ruined car.