At least 46 died Wednesday in and around the city of La Plata, Gov. Daniel Scioli said. Six deaths were reported a day earlier in the nation's capital.
Many people climbed onto their roofs in the pouring rain after storm sewers backed up. Water surged up through drains in their kitchen and bathroom floors, and then poured in over their windowsills.
"It started to rain really hard in the evening, and began to flood," said Augustina Garcia Orsi, a 25-year-old student. "I panicked. In two seconds, I was up to my knees in water. It came up through the drains — I couldn't do anything."
The rains also flooded the country's largest refinery, causing a fire that took hours to put out. The La Plata refinery suspended operations as a result, and Argentina's YPF oil company said an emergency team was evaluating how to get it restarted.
"Such intense rain in so little time has left many people trapped in their cars, in the streets, in some cases electrocuted. We are giving priority to rescuing people who have been stuck in trees or on the roofs of their homes," Scioli said.
But many complained that they had to rescue themselves and their neighbours as cars flooded to their rooftops and homes filled with up to two meters (six feet) of water.
"We lost family heirlooms, appliances, clothing," said Natalia Lescano, who escaped with her family to a friend's house on higher ground.
President Cristina Fernandez arrived by helicopter in Tolosa, a La Plata neighbourhood where she grew up and where her mother was among those evacuated. She announced security measures to combat vandalism, help for identifying the dead, and three days of national mourning for the victims.
She was then was surrounded by her mother's neighbours, in a rare uncontrolled encounter with everyday citizens. Some hugged and thanked her. Others complained angrily and shouted at her to "go away."
"It's a disgrace," Miguel Garcia, a 58-year-old shopkeeper, said earlier. "They need to govern. My mother-in-law is disabled. We had to carry her up to the roof, and then we had to rescue ourselves because no ambulance would come."
The coast guard finally reached the Bozzano family on their rooftop an hour before dawn. By then, their car had floated away and everything inside the house was destroyed.
"We were trapped inside the house and couldn't get out because of the water pressure. Finally we were able to open a door and escaped to the roof. That's where we spent the night," Mauricio Bozzano said.
The heaviest rain — almost 16 inches (400 millimeters) in just a few hours, beating historical records for the entire month of April — hit provincial La Plata overnight. A day earlier, the capital of Buenos Aires was hit hardest.
About four more inches (100 millimeters more) of rain were expected before the bad weather passes on Thursday, the national weather service said.
At least 2,500 people were evacuated from their homes to about 20 centres in the La Plata area, which is about 37 miles (60 kilometres) southeast of Argentina's capital.
The flooding threatened to ruin food supplies across La Plata's metropolitan area, which has nearly 1 million people.
It also closed the private Spanish Hospital, a complex that covers an entire city block, after waters rushed into the basement, cutting power and destroying X-ray machines and other diagnostic equipment.
"We're sending away all the patients and the hospital will be closed for several days," said Sebastian Sambron, one of the hospital's top officials. "We're telephonically cut off, and without power since last night. The hospital is collapsed."
National Planning Minister Julio de Vido estimated that 280,000 people remained without power across the city and surrounding province of Buenos Aires, where most Argentines live.
"Our job is focused on restoring service, but we're going to wait until the equipment dries to guarantee the safety of the electricity workers, because we don't want any deaths," De Vido said.
YPF said no injuries were caused by the refinery fire, which it blamed on "an extraordinary accumulation of rainwater and power outages in the entire refinery complex." The impact on Argentina's chronically short fuel supplies wasn't immediately clear.
The six killed in Buenos Aires included a subway worker who was electrocuted and an elderly woman who drowned inside her home. Many evacuees slept in their cars overnight, and still had standing water in basements, parking lots and storage rooms.
The governments of Fernandez and Buenos Aires Mayor Mauricio Macri sought to blame each other for the chaos, and the nation's divided media focused their coverage in ways that put one side or the other in the worst light.
Macri said Wednesday that the only solution is for the constantly warring governments to work together on expensive and long-term public works projects, creating huge underground drainage pipes to carry increasingly common torrential rains out to the Rio de la Plata.
"Facing the magnitude of what we've lived through, I insist that public works are what will change this story," Macri said, describing one such project that was achieved through regional co-operation and a World Bank loan — the kind of borrowing that Fernandez has sought to avoid.
"We need to do the same with all the waterworks that are needed in the city, in greater Buenos Aires and in the province of Buenos Aires," Macri said.
Associated Press Writer Cristian Kovadloff in La Plata and Almudena Calatrava, Debora Rey and Michael Warren in Buenos Aires contributed to this story.