Some keen bargain-hunters started queuing before dawn to ensure they were first in line when the shops opened and some stores laid on food and entertainment to entertain those waiting in the cold.
A busy Boxing Day was predicted in Glasgow with queues at Silverburn shopping centre from 5am.
The centre enjoyed a 20% year-on-year boost in footfall over the week leading up to Christmas.
Retailers have now slashed prices by as much as 83% in a bid to attract more buyers.
General manager David Pierotti said: "It's tradition for Boxing Day to offer some fantastic deals and we are expecting our customers to start snapping them up as Next opens at 6am."
Aberdeen's Union Square shopping centre general manager Ryan Manson said: "There are some great deals on in the centre and shoppers will be keen to snap them up and we are anticipating them arriving early this morning.
"Traditionally when people talk about Boxing Day sales they think of clothes but now this extends to the best gadgets, phones, homewear and toys."
Union Square's branch of clothes store Next opened at 6am to deal with expected high demand for its sale, while Marks and Spencer opened at 8am.
In London, hundreds of shoppers surrounded Selfridges, where queues snaked around the huge shopping emporium on London's Oxford Street.
There were separate queues for different brands and a significant security presence was in place to manage the crowd before doors opened at 9am.
Then, bargain-hunters surged in, filling every possible space on the shop floor.
The crowd, a significant number of whom were Chinese, then began to form queues at the various designer concessions within the store.
Long queues formed at Gucci, Mulberry and Prada, as the shoppers battled to snap up the good deals.
Eyal Keden, 40, from Israel, is in London on holiday and had been queueing since 6.30am.
"It's amazing. We don't have this in Israel," the engineer said.
Mr Keden said he and his partner were on the lookout for bags and shoes and hoped to get a few bargains.
He said they did not come to London specifically for the sales, and that they were just "a bonus".
Also in the queue was Wang Tianyi, 21, a student from China who is studying in Liverpool, and his girlfriend Sun Yeting, also 21, who is on holiday visiting him.
The couple had been queuing since 4am, and were rather cold four-and-a-half hours later.
"We don't prepare very well," Mr Tianyi joked, adding: "I've heard it's crazy."
The couple said they hope to pick up lots of gifts for their family, and said they will go straight for the luxury goods.
"We will buy the things we like," Mr Tianyi said.
Harrods also kicked off its winter sale and offered mugs of hot chocolate, smoked salmon canapes and blankets to keep people warm in the queue, which was serenaded by a string quartet and a performance by street magician Dynamo.
Department store Liberty offered designer goods at a fraction of their usual price, while both Next and Marks & Spencer boasted discounts of up to 50% on sale items.
Online sales started on Christmas Eve and millions are believed to have already snapped up goods from the comfort of their own homes.
Some 117 million visits to retail websites were estimated to have been made yesterday in the UK, with today's figure set to rise to more than 118 million.
Some retailers were hoping brisk business at the sales would make up for disappointing pre-Christmas takings.
Experian said that between December 24 and 26, some 45 million hours would be spent shopping online.
By the end of December, it is estimated there will have been three billion visits to retail websites in the UK, which will be the first time that figure has been reached in a single month.
James Murray, digital insight manager at the information services company, said: "Christmas 2013 has consistently outperformed 2012 on virtually every single shopping day this December, with online visits from Christmas Eve through to Boxing Day up from last year, a record-breaking Cyber Monday and the emergence of the even busier Middle Cyber Monday.
"Shopping habits are changing, with Christmas Day becoming a significant shopping day during the period.
"As a result, we anticipate a more sustained shopping pattern during this period, moving away from the traditional peaks and troughs usually evident during the holiday season."
Bluewater in Kent predicts more than 800,000 people will come through its doors between today and New Year's Eve.
Robert Goodman, Bluewater's general manager, said: "We are ready for a bumper Boxing Day this year. With the strongest retail offer to date, with 50 new brands launching with us this year, we are expecting visitor numbers to be on a par with Boxing Day last year.
"This will begin what is likely to be our busiest six days of the year."
Meanwhile, at Cabot Circus in Bristol shoppers began queuing at 6am for the chance to grab a bargain.
Stephanie Lacey, centre director, said: "Shoppers from across the South West braved the cold weather and came out in force at Cabot Circus today as the Boxing Day sales got under way.
"Queues began forming as early as 6am as shoppers made a beeline for stores including Next, Harvey Nichols and House of Fraser.
"We're expecting designer labels and brands to be very popular.
"We could see 100,000 shoppers at Cabot Circus today, and the pattern should continue for the next few days."
At St David's shopping centre in Cardiff, retailers were expecting 150,000 visitors, with sales tipped to reach £2 million.
Centre director Steven Madeley said: "The Boxing Day sales have got off to a flying start in Cardiff this morning, with thousands of Welsh shoppers descending on St David's in the hunt to bag the best sales bargains.
"Our department stores and high-end retailers are looking extremely busy as shoppers make a beeline for those luxury items and designer names at reduced prices.
"We are expecting up to 150,000 shoppers at St David's today and sales could reach £2 million.
"Tomorrow is likely to be even busier as stores open for longer. We think up to 200,000 shoppers could visit us on Friday.
"The sales period is equally busy as in the run up to Christmas itself. We should welcome one million customers over the next week."