A curfew in ChileConcepcion, has been extended until (1500 GMT) as troops struggle to contain looting after the earthquake.
Dozens of people were arrested after looters fought over goods and set fire to a department store, officials say.
The authorities have announced the setting up of a special air route to deliver aid from the capital, Santiago, to Concepcion, 430km (270 miles) away.
Saturday morning's 8.8-magnitude earthquake killed at least 723 people.
The deteriorating security situation in Concepcion comes despite the influx of thousands of troops to reinforce local police.
Many of the city's 500,000 inhabitants are short of food and have seen their water and electricity supplies cut off.
But lorry after lorry loaded with water, food and mattresses is being held up by the military until the curfew is lifted, reports the BBC's Andy Gallacher from the checkpoint on the outskirts of Concepcion.
The main highway in the region is twisted and bent out of shape, but the route remains open, our correspondent says.
Security seems to be the biggest issue holding up rescue efforts, he adds.
Some residents quoted by the Reuters news agency said they were organising groups to defend their property.
President Michelle Bachelet, condemning "pillage and criminality", has sent 7,000 soldiers to the region.
"I want to call to the people's conscience. We must all work together," she said.
Meanwhile, rescuers searching the rubble of a collapsed apartment building in the city in which dozens are feared trapped say they have heard signs of life and are attempting to reach survivors.
Reports are beginning to emerge of the scale of the devastation in other areas.
A BBC team that reached the town of Curico, between Santiago and Concepcion, found widespread destruction.
Up to 90% of the mud-and-wood buildings in the town's historic centre had been destroyed, and a hospital badly damaged, our reporters said.
However, food and water was being distributed and the situation was comparatively calm, they added.
Some coastal towns and villages were hit by giant waves after the earthquake.
In the fishing village of Constitucion, the mayor said the seafront and centre had been "completely destroyed".
The government admits that its attempts to provide aid swiftly have been hampered by damaged roads and power cuts. The air bridge between Santiago and Concepcion will help the authorities send more than 300 tonnes of aid, including 120 tonnes of food, to the worst affected area of the country.
International aid has begun arriving. Neighbouring Argentina is flying a field hospital over the Andes to Chile and has pledged half a million litres of much-needed drinking water.
Brazilian President Luiz Inacio Lula da Silva flew to Santiago and offered his nation's support, while US Secretary of State Hillary Clinton will arrive shortly.
Earlier, Mrs Clinton said President Bachelet had initially told her Chile could cope with the disaster, but when the scale of it became apparent she changed her mind.
The Chilean government had requested field hospitals, water purification units and satellite phones, she added.
"One of their biggest problems has been communications," Mrs Clinton told reporters. "They can't communicate into Concepcion and some of the surrounding areas."
After touring the disaster zone, President-elect Sebastian Pinera said the situation was worse than he had expected.
"When we have a catastrophe of this magnitude, when there is no electricity and no water, the population... starts losing the sense of public order," he said.
About two million Chileans are believed to have been affected by Saturday's earthquake, the seventh most powerful on record and the worst disaster to befall Chile in 50 years.
The epicentre of the quake was 115km (70 miles) north-east of Concepcion and 325km south-west of the capital Santiago.
About 1.5 million homes in Chile have been damaged. Most of the collapsed buildings were of older design - including many historic structures.
One US risk assessor, Eqecat, has put the cost of repairing the damage at between $15bn and $30bn (£9.8bn - £19.6bn) or 10-20% of the country's gross domestic product. - BCC