Record discounts by retailers are predicted for the Boxing Day sales as they try to attract shoppers being squeezed by the credit crunch.
Sales in store began on Boxing Day
Kevin Hawkins, director general of the British Retail Consortium, said retailers need to recoup low Christmas profits, particularly from white goods.
Some firms, such as Debenhams on Oxford Street in London, are offering sales discounts of up to 70%.
An estimated 3.57 million began their sales shopping online on Christmas Day.
The British Retail Consortium said deep discounts had attracted a rush in the 10 days before Christmas, and as a result, sales will be up on 2006 and retailers are due to beat last December's year-on-year growth.
But they said although the heavy discounting had tempted shoppers in, retailers would have seen "margins squeezed in a crucial month".
Mr Hawkins told BBC Radio 4's Today programme: "It's not inconsistent to have retailers whose profits are falling and yet their sales are going up - that's one of the problems of discounting.
"You offer big discounts to get your goods sold through, because you don't want a lot of seasonal stock left over, but at the same time it's not doing your margins any good - they might be falling.
"But it's worthwhile getting rid of that unsold seasonal stock and I think that's what we're going to see a lot of over the next three days."
The number of shoppers in London and Birmingham was already beginning to grow during mid-morning.
Sheffield and Newcastle were also seeing large numbers of bargain hunters on the streets.
Loretta Frazer, manager of Selfridges in the Bullring shopping centre in Birmingham, said 500 people waited outside the shop's closed doors for the 0800 GMT opening.
She told BBC News: "The sales are recognised as a great bargain opportunity...within the first hour we had taken 65% of our sales on our designer handbags."
But she also said that the coming year would be tough, and that retailers would have to be very competitive to attract customers.
In Edinburgh shops were also doing brisk business.
Andrew Cronie, manager of the Gyle Shopping Centre in Edinburgh, told BBC News: "It's been a very busy day with people queuing up long before our doors opened.
"We anticipate there will be a surge of customers looking to take advantage of the sales over the next few days."
However for the Castle Court shopping centre in Belfast, the biggest in Northern Ireland, business was solid but not frantic.
Stuart McConnell, security manager at the centre, said: "I can't say it's been very busy, but certainly there were people queuing up at some shops prior to us opening up at 11.
"For this time of the morning it's fairly hefty, but nothing spectacular."
He said the run-up to Christmas had been extremely busy, with 2.1 million people passing through the centre in the previous five weeks, with clothes, electrical goods and computer games doing particularly well.
Business as usual
Mr McConnell said next year's business was "a $60m question" but that the centre was optimistic.
In Glasgow the story was much the same.
Asked whether the credit crunch has had an impact, Peter Beagley, general manager of the Braehead Shopping Centre, said trade was busy but no different from last year.
"We've been as busy as we were the Christmas before, so it doesn't seem to have had an impact on us," he said.
However some people avoided the shopping centres and high streets by starting shopping over the internet on Christmas Day, where online sales were posted from one minute past midnight on Christmas morning.
High-street retailers have reported record online sales of £52m - which they say is their highest online Christmas sales figure to date.
The Marks and Spencer website reported busy traffic soon after its online sale started at midnight on Christmas Day.
While clothing were popular purchases, other online shops predicted TVs and laptops would be snapped up.
A Post Office survey of 2,042 people suggested one in five Christmas Day web surfers would buy something online.
A spokeswoman for M&S said the company was expecting at least 200,000 shoppers online for Christmas.
Bucking the trend for the rush to attract customers is John Lewis - it starts its sale on Thursday.