High streets and shopping centres up and down the country are expected to be swamped by millions of bargain-hunters as the traditional Boxing Day sales get under way.
In London's West End, people queued overnight on Oxford Street in preparation for the stores opening.
Oxford Street, London's retail mecca, will see several hundred thousand customers pass through.
Many high street retailers have started their sales two days early this year
It was bustling this morning with shoppers from across the country and thousands of tourists, many from China.
One man said the prices in the United Kingdom offered better value for money than in China and fighting for bargains in Oxford Street was an experience.
Scenes of shopping mayhem in London were replicated across the country.
At Birmingham's Bullring Shopping Centre, thousands were ready and waiting from 12.20am for the off.
Popular picks were the Next Boxing Day Sale, and the promise of knock-down prices at both Debenhams and Selfridges department stores.
Tim Walley, Bullring general manager, said: "Today we expect over 200,000 customers to flock to Bullring all eager to pick up some bargains."
It was a similar story at Leicester's Highcross Shopping Centre, with shoppers queuing from 1.30am.
More than 90,000 shoppers are expected through the centre's doors when John Lewis starts its post-Christmas sales.
Jo Tallack, general manager, said: "We welcomed more than 1.5 million shoppers to Highcross in the run up to Christmas and expect them to continue coming through the doors for post-Christmas bargains between now and the near year.
"We expect to see more than 90,000 shoppers today and another busy day tomorrow when our flagship retailer, John Lewis, opens for its annual clearance event."
In December, sales at the centre increased 3% on the year, with an estimated two million shoppers through its doors for this month alone.
In Bristol, hundreds of people queued from the early hours of this morning to be among the first through the doors of Cabot Circus' Next store.
Centre director Kevin Duffy said a long line of eager bargain hunters greeted Next staff as the clothing chain opened at 6am in Bristol for the start of its famous Boxing Day sale.
He said: "There were a couple of hundred people waiting at 6am, and three hours later the queues are still there waiting their turn to get in.
"Several other stores within the shopping centre opened at 8am, and there were lots of people queuing there, too."
Duffy said around 180,000 people are expected to visit Cabot Circus today and tomorrow, with an estimated 100,000 of those waiting the extra day to pick up deals.
He said: "I think some people are still coming back from holidays or time spent visiting family today, so we are expecting slightly more people through the doors tomorrow.
"That seems to be the way things happen now, people are happy to wait until December 27."
But many online retailers tried to stay one step ahead of the competition by offering heavy discounts as early as Christmas Day.
Amazon's UK website said it had seen sales on Christmas Day increase by 263% over the last five years, partly due to the growth in home broadband and the popularity of tablets and smartphones.
The retailer is launching further offers today, which include clearance offers and "lightning deals" for a limited time and quantity of stock.
According to MoneySupermarket.com, shoppers in the UK are set to spend a total of £2.9 billion in the Boxing Day sales.
A poll for the website found almost four million Britons (8%) plan to head to the high street on Boxing Day in addition to more than five million (10%) who will be searching online.
Analysts Experian predict that Christmas 2012 will be the "biggest and busiest ever" for online retailers in the UK, with visits to retail websites expected to reach 126 million today, up 31% on 2011.
They believe consumers will spend £472.5 million on Boxing Day bargains.
Experian's digital insight manager James Murray said: "Christmas 2012 is on track to be another record-breaker for online retail, outstripping 2011 on all fronts.
"The current market trends suggest that in the UK, Boxing Day will be the biggest day for online retail, with an estimated 126 million visits to online retail outlets and a massive 17 million hours spent online shopping on this day alone."
But figures from one survey suggested that almost half (47%) of those questioned were not planning on buying anything in the post-Christmas sales, comparison website Pricerunner said.
Meanwhile, high-street spending was "acceptable but not exceptional" this festive period, according to the British Retail Consortium.
As more shoppers flocked to the web rather than the shops to buy presents, high-street retailers faced the threat of closure, business recovery group Begbies Traynor warned.
Almost 140 firms were in a critical condition in the fourth quarter, meaning they are on the brink of collapse, while more than 13,700 were in "significant" distress - up 35% during the three months to December 17, the Begbies Traynor report said.
But the BRC's head of media and campaigns Richard Dodd said poor accessibility on high streets, a lack of parking and weak consumer demand were to blame rather than an increase in online shopping.
But he acknowledged that some high-street retailers would "undoubtedly" fail after Christmas.
"Retail sales over the weekend have been up to expectations but expectations were relatively modest," said Mr Dodd.
"Christmas will turn out to be acceptable but not exceptional."
He went on: "There are a lot of myths around online retail.
"Ten per cent of overall retailing over the year comes from online shopping and actually it presents lots of opportunities for the retail sector."