Retailers are hoping to make up for a disappointing Christmas with the launch of the New Year sales.
London's Oxford Street may have its busiest day of the year
More than 2,000 people queued outside Selfridges's flagship store in London - some as early as 5am - in an attempt to get some of the hottest bargains.
More than 27,500 people passed through the doors of Birmingham's Bullring during the first hour of trading.
More major chains are now opening on Boxing Day in a shift from sales traditionally begining on 27 December.
Up to 75,000 shoppers are expected to shop at Bluewater in Kent on Boxing Day. Some of the 330 stores at the centre have discounted stock by as much as 70%.
While many major retailers have opened the doors of their Scottish branches for many years, English and Welsh outlets stayed shut. But since 2003, when three major regional shopping centres opened on Boxing Day, more and more businesses have followed suit.
A spokesman for supermarket chain Somerfield said it would be opening most of its 900 stores.
"It is the first time they've all opened on Boxing Day," he said. "We run smaller convenience stores, and as we are in that sector we have found our customers want easy access to top up on things they have run out of."
A spokeswoman for Manchester's Arndale Centre said business was brisk, with as many as 70,000 people expected during the day.
But she said: "Tomorrow is going to be our biggest day of the year, with 250,000 people expected to flock to our doors."
The centre said it would open its doors at 0300 GMT, with retailer Next starting its sale at 0500 GMT and most other reatilers at 0900 GMT.
Harry Millar, general manager of the Buchanan Galleries shopping centre in Glasgow, said business was going well.
"It is going very well and is very lively. There are some good deals to be had and it seems to be up there with previous Boxing Days."
Alison Light, operations manager of Aberdeen's Bon Accord St Nicholas Centre, said at least 45,000 people were hoped to visit the centre during Boxing Day.
Natasha Burton, of retail market analysts FootFall, said the sales followed heavy discounting in the run-up to Christmas.
"It is a very important time for the retailers but the key thing will be how they make these sales stand out to attract people in," she said.
Some shoppers waited outside Selfridges from 5am
Footfall said the number of visits made to high street shops before Christmas fell 40% compared to the previous year.
It said fewer people bought last-minute presents - they were down 43% - but said this was partly due to Christmas Eve falling on a Sunday, with limited opening hours.
The New West End company, representing stores on the main London shopping thoroughfares of Oxford Street, Regent Street and Bond Street, said it expected some stores to have their busiest shopping day of the year.
Kevin Hawkins, director of the British Retail Consortium, said sales were a good way of stores clearing excess stock.
He said: "There are always some good bargains to be had. It is just whether you have got the patience and the sheer grit to go hunting."
Four out of five people will buy something in the winter sales, a survey has suggested.
The HSBC Bank sales survey found two-thirds of men interviewed (65%) wanted to save money on a particular item, while three-quarters of female shoppers said they would simply be looking for a bargain.
More than half of women (52%) interviewed said they would hunt out a luxury item they would not normally be able to afford.