The numbers of police officers and soldiers on the streets of quake-affected cities like Talcahuano have been bolstered
Chile's president has appealed for calm in the earthquake-ravaged city of Concepcion, vowing a stern response to any renewal of looting and violence.
Michelle Bachelet says 14,000 troops are now in the region, after dozens of looters were arrested.
As night fell, curfews were imposed across four major urban centres in Chile, including an 18-hour curfew in Concepcion, one if its largest cities.
Some half a million people are homeless in a city now under military control.
The death toll from the 8.8-magnitude quake now stands at 795, officials say, but emergency workers also say 19 people are still unaccounted for.
One mayor, from Hualpen, near Concepcion, said many on the streets were more terrified of crime than aftershocks.
"The thugs have taken over the city. Now we are not afraid of the earthquakes, we're afraid of the criminals," Marcelo Rivera told a Chilean radio station.
Armoured vehicles have been stationed at strategic points across Concepcion and armed soldiers patrol the streets.
Groups of residents are reported to have gathered together to form vigilante groups to confront would-be looters.
A special air route is being set up to deliver aid from the capital, Santiago, to Concepcion, 430km (270 miles) away.
But security in the city remains a key concern after shops and homes were looted on Monday and police made a large number of arrests.
The deteriorating security situation in Concepcion comes despite the influx of thousands of troops to reinforce local police.
"We can say that, according what we've been told from the area, the situation in Concepcion is under control today," President Michelle Bachelet said on Tuesday.
But, she added, authorities would take any "necessary measure" to stop renewed looting.
"Our principle objective is to go and help people tackle the emergency in the disaster zone.
"I want them [looters] to understand this and that they'll receive rigorous legal action. We will not tolerate such actions."
Many of the city's 500,000 inhabitants are short of food and have seen their water and electricity supplies cut off.
Aid agencies have yet to reach Concepcion, reports the BBC's Andy Gallacher, who has reached the city, where many people are still awaiting water, food and mattresses.
However, at least two police officers appear to be posted on every corner in the city centre, our correspondent says.
Some residents quoted by Reuters news agency said they were organising groups to defend their property.
Reports are also beginning to emerge of the scale of the devastation in other areas.
AT THE SCENE
Andy Gallacher, BBC News, near Concepcion
It has taken us about 15 hours of solid driving to get to the outskirts of Concepcion. The main highway is ripped and twisted all the way down from the capital, Santiago.
At a military checkpoint, there are rows and rows of lorries carrying food, fresh water and other emergency supplies.
It appears that there has been a complete breakdown of law and order in Chile's second city.
Some of the communities on the coast near here have meanwhile not yet even been heard from. They were first hit by the earthquake, and then swept away by the tsunami.
Up to 90% of the mud-and-wood buildings in the historic centre of Curico had been destroyed or damaged, and a hospital badly damaged, BBC reporters said.
Some coastal towns and villages were also hit by giant waves after the earthquake, with some reported to have been completely destroyed.
Reports from the town of Pelluhue suggested that a series of tsunamis swept through what was a tranquil seaside resort, destroying houses and claiming many lives.
The government admits that its attempts to provide aid swiftly have been hampered by damaged roads and power cuts.
The air supply route 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, as did US Secretary of State Hillary Clinton.
Hillary Clinton holds joint conference with Michelle Bachelet
Mrs Clinton took a consignment of satellite phones with her to Santiago after the Chilean government requested communications equipment alongside field hospitals and water purification units.
"We stand ready to help in any way that the government of Chile asks us to," said Mrs Clinton, adding: "The United States will be there to help when others leave."
After touring the disaster zone, Chilean President-elect Sebastian Pinera - who takes office on 11 March - 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.
AID PLEDGES (in US$)
European Union: $3m
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.