Chilean authorities have imposed an overnight curfew in the quake-damaged city of Concepcion in an attempt to prevent looting, after hundreds of people ransacked stores for food.
Police said on Monday they had arrested dozens of people for looting and breaking the curfew, while some reports said forces had fired tear gas to stop people taking food from shops.
The government faces the stiff challenge of providing aid to thousands of people left homeless across the country after an 8.8 magnitude earthquake hit on Saturday, killing at least 708 people.
Michelle Bachelet, the country's president, has promised deliveries of food, water and shelter for thousands living on the streets.
"We are confronting an emergency without parallel in Chile's history," she said, adding that a "growing number" of people were recorded as missing.
Lucia Newman, Al Jazeera's correspondent in Concepcion, said there is "a feeling of desolation" in the city.
"There's no water, no food, no fuel, no electricity. There's been wide looting ... we saw people looting a flour mill.
"At night, the neighbours have been arming themselves with bats, in some cases even with guns. People are afraid that groups of gangs will try to go into their apartments," she said.
The United Nations has said it will rush aid deliveries to Chile in response to a request from the government.
Television images showed residents carting food and electronics from damaged stores
"We are prepared to provide assistance,'' Elisabeth Byrs, a UN humanitarian spokeswoman, told the AP news agency.
"It could be quite fast, given that our experts are on standby and were alerted in the region."
The International Red Cross said volunteers were providing first aid in areas hardest hit, and that it was appealing for donations within Chile.
Bachelet has said she expects the death toll to continue to rise as the scale of the devastation became clear from coastal towns and villages engulfed by giant waves.
Some coastal towns were almost obliterated, after being first shaken by the quake and then slammed by sea waves.
Angry survivors have taken out their frustration on firefighters delivering water in Concepcion, damaging their vehicles.
"We don't have water or anything. No one has appeared with help and we need more police to keep order," a 78-year-old woman in the badly hit city of Talca, told the Reuters news agency.
"There are many people here who are robbing," she said.
'Out of control'
Jacqueline van Rysselberghe, the mayor of Concepcion, said the situation was getting "out of control" due to shortages of basic supplies.
But she warned that the looting was "totally unjustifiable".
"We've got a very complicated situation and the people feel very vulnerable," she told local radio.
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Saturday's quake also triggered a tsunami that killed at least four people on Chile's Juan Fernandez islands and caused serious damage to the port town of Talcahuano.
More than 60 aftershocks of magnitude 5 or greater were reported in the hours after the quake.
The US Geological Survey (USGS) said the earthquake struck 92km northeast of Concepcion at a depth of 63km at 3:34am local time (06:43 GMT) on Saturday.
Aid organisations and government officials in Britain and the US have offered to help Chileans in their efforts to rebuild their country.
Barack Obama, the US president, has said the US has resources in position to deploy should the Chilean government ask for help.