Argentine President Cristina Kirchner, who spent much of her childhood in the worst-hit city of La Plata, declared three days of national mourning on Thursday.
Meanwhile, the death toll from the flooding rose to 57, with thousands being forced to flee their homes and others living on their roofs.
In a two-hour period from late Tuesday into Wednesday, some 40 centimeters (16 inches) of rain fell in La Plata. Argentina's Deputy Security Minister Sergio Berni said on Thursday that the final number of dead was yet to be assessed.
"We are carrying out the final count because there are still about 20 people who have not been found," said Berni.
Some 750 provincial police were sent into the affected areas after reports of increased crime. During the night, a store and an elementary school were looted. The national government sent in army, coastguard, police and social welfare workers.
Buenos Aires provincial governor Daniel Scioli noted that La Plata, in particular, had suffered "a tragedy without precedent and of the most serious consequences."
"Some people just didn't have time to escape this deadly trap, with such a large volume of water in such a short period of time."
"The humanitarian question comes first," said Scioli. "The material questions will be resolved in time... I will apply the necessary resources."
Warding off disease
Four mobile hospitals were activated with water, food and clothing being distributed at shelters. Provincial Health Minister Alejandro Collia said that areas were being sprayed to kill mosquitoes and that hepatitis vaccines were being administered.
The majority of the dead, at least 49 people, were in La Plata, where flood waters reached two meters in some places. President Kirchner surveyed the area by helicopter on Wednesday. Six others died in the capital, Buenos Aires, while another two were killed in surrounding suburbs.
Pope Francis, former Archbishop of Buenos Aires, on Thursday said he was deeply saddened and expressed "his closeness to the dear people of Argentina."
Some 1,500 people were evacuated from La Plata on Wednesday. More than 300,000 people in the city remained without electricity on Thursday, along with some 290,000 people in the capital.
rc/kms (AFP, AP, dpa, Reuters)