Retailers expect the day after Boxing Day to be the busiest of the year as a fresh wave of bargain hunters brave the queues to pick up cut-price items.
The sudden drop in prices could also hurt retailers' earnings
A number of companies including Marks and Spencer, Next and John Lewis have opened after closing for Christmas.
The Trafford Centre in Manchester expects 140,000 customers, up from 100,000 on Boxing Day. London's Brent Cross shopping centre expects 100,000.
But fears remain that heavy discounts will hurt retailers into 2008.
"The first two or three days after Christmas are very important," said Martin Davies of research firm Experian, which compiles a Retail Footfall Index (RFI)
"But already in the days before Christmas, the surge of shoppers on to the High Street looked like sales shopping and not pre-Christmas buying."
High Street shops have been slashing prices to clear old stock that they have not been able to shift as many cash-strapped shoppers stayed away, worried about their finances in the face of more expensive household bills.
The sales seemed to have some success in tempting people from their homes, with figures from research firm Footfall showing a 25.1% increase in shopper numbers on Boxing Day compared to the same day in 2006.
The British Retail Consortium (BRC) is predicting that sales will continue well into next year as shops compete heavily for business.
This is bad news for retailers as discounting erodes already thin margins, analysts say.
"Retailers who haven't done the business they expected to before Christmas will be very keen to cut their losses and shift whatever seasonal stock they've been left with," said BRC director general Kevin Hawkins.
"With a challenging first half of the year virtually guaranteed for all consumer-facing businesses, retailers will be discounting well into 2008."
John Lewis may be one of the few retailers to buck the trend.
Andy Street, the department store chain's managing director, told the BBC that its profit margins would not be squeezed in the post-Christmas clearance sale and that earnings would rise.
"We have probably won market share in this tough Christmas," he said.
In London, the throng of consumers was powerful, with more than 10,000 people reckoned to have visited Brent Cross shopping centre during the first hour of trading on Thursday.
According to Paula Figgitt, a spokeswoman for Brent Cross, the figure is expected to hit 100,000 by the end of the day.
About 3,000 people had gathered outside the Bluewater mall in Kent from 0300 this morning.
Property manager Tim Holland said emergency car park measures would be put in place by the afternoon, with the number of visitors to Bluewater expected to peak at close to 200,000 at about 1400.
Meanwhile, in the Birmingham area, a number of women were injured in the rush for bargains at the retailer Next, the West Midlands Ambulance Service said.
The ambulance service said it attended three different Next branches after a woman passed out, another suffered a hand injury and a third experienced breathing problems. A fourth woman was said to have fainted.
The branches involved were at the Bull Ring in Birmingham, the Fort Shopping Centre and a Next branch at Axletree Way in Wednesbury.
"Our staff were somewhat surprised to get so many calls to different branches of the same clothing company," an ambulance spokesman said.
He added that people queuing for bargains should "wear suitable clothing to keep warm and either have had something to eat and drink before going or take it with you, so that you don't become dehydrated or hungry".