Soldiers patrolled the streets of the city of Concepcion after mobs set fire to stores and engaged in mass looting, sidetracking authorities' attempts to rescue survivors.

A curfew imposed on Monday was extended after mobs continued to set fire to shops and businesses.

President Michelle Bachelet said there would be 7,000 troops in Concepcion and other towns by Tuesday.

Armed civilian groups were also on the streets, setting up fires in areas darkened by power cuts to try to keep looters away from empty homes.

One person was reported killed as troops and police made 160 arrests.

The quake zone has continued to be shaken by aftershocks. Over 120 tremors with a magnitude greater than 5.0 have hit the region since the quake struck.

Deputy interior minister Patricio Rosende said Concepcion was a dangerous city.

"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.

Violence has also been reported in the seaside villages that were swamped by massive waves following the quake.

More than 300 bodies were reportedly found in the fishing village of Constitucion, where hundreds of homes were swept away.

Meanwhile, aid pledges continue to pour in from around the world after the government appealed for international help. The death toll currently stands at 723.

Outgoing president Ms Bachelet has requested mobile bridges, field hospitals, satellite phones, electrical generators, disaster assessment and co-ordination teams, water purification systems and field kitchens, UN officials say.

Some two million Chileans are estimated to have been affected by Saturday's massive tremor and the tsunamis that followed.

The quake, the most powerful to hit the nation in 50 years, sent shockwaves out from the epicentre 70 miles from Chile's second city, Concepcion.

Buildings and roads collapsed and 500,000 homes have been left severely damaged.

Six aid workers died when a plane carrying them to Concepcion crashed.

The team was on its way to help organise accommodation for those left homeless by the disaster.

Prime Minister Gordon Brown and the Queen have sent messages of sympathy to President Bachelet as millions of pounds worth of aid was announced for the stricken country.

About 65,000 Britons visit Chile each year, according to the country's tourist authority.

Relatives of two British couples missing since the quake have spoken of their relief after it emerged their loved ones were alive and well.

Fears had been growing for Andre Lanyon and Laura Hapgood from Guernsey and Kirsty Duff and Dave Sandercock from Scotland who all failed to contact home.

But on Monday night they rang relatives after reaching the Chilean capital Santiago.

Mr Lanyon's mother, Valerie, 55, said: "I haven't slept for three days and I certainly won't be getting any tonight. We're so excited and relieved.

"He sounded fine. They had just arrived in Santiago when they called."

Mr Lanyon, a 29-year-old firefighter, was surprised to learn he had sparked national headlines, his mother said.

Ms Duff, from Stonehaven, Aberdeenshire, and Mr Sandercock, from Edinburgh, both 25, arrived in the country three weeks ago.

The families of Ms Duff and Mr Sandercock released a joint statement saying they were "happy to announce that they have arrived safely".

It added: "We have been in direct contact with Kirsty and Dave and can confirm that they are safe and well.