Outside Pisco, a mob blocked the road close to an air force base where the humanitarian aid effort is being centralised.
Local radio reported that truck drivers did not resist when the residents swarmed their vehicles and that police from a nearby station had not intervened.
Teresa Bo, Al Jazeera's correspondent in Pisco, said: This is not normal looting. People are just trying to find food, water and clothes."
Hours later another group tried to raid a convoy of trucks carrying emergency supplies close to the provincial capital of Ica.
Police are also working to track down more than 600 inmates who escaped from a prison in Chincha after the earthquake toppled the prison walls.
Lacking official figures, news media estimate the number of people affected by the quake at anywhere from 60,000 to 200,000, with many tens of thousands believed to have lost their homes.
Unidentified bodies still line the streets of Pisco and other towns in the region.
Luis Gonzales Posada,the head of Peru's congress, said that Pisco "seems like it was bombed" after he visited the town.
He said: "We have been evaluating the material damages and the urgent needs of the population. We have here thousands of people we need to feed."
Guillermo Merino, who heads the town's firefighters, said the situation in Pisco "is made worse by the lack of tents, food, water and medicine".
Even though the days remain warm, temperatures are now dropping to 10C at night, hitting countless poor people who lost their adobe hut homes which crumbled in the quake.
Victor Ortega, 65, said: "The nights are horribly cold, and it also rains. It is worse than having been bombed in a war."
The Peruvian navy announced that it was sending two ships with drinking water to Pisco and a hospital ship to treat the wounded.
Alan Garcia, the country's president, has thanked the international show of solidarity for the earthquake.
Garcia said he had "spoken with a great many heads of government who were shocked by the earthquake and are coming forth with assistance for the victims".
He called the international effort to send aid "the globalisation of good feelings".