Police have criticised supermarket chains after stores were over-run by bargain-hunters overnight as the US Black Friday retail phenomenon hit the UK.
Websites also crashed under the strain of online shoppers as sales began at midnight, with hordes descending on stores in a rush to pick up goods ahead of Christmas.
Officers were called to at least four supermarkets in London alone because of over-crowding while shoppers took to Twitter (Xetra: A1W6XZ - news) to describe "carnage" in stores, with some reports of people being stripped of cheap TVs and other goods by rivals in the scramble.
Queues were seen up and down the country and frustrations boiled over.
Greater Manchester Police said on its Twitter feed: "Keep calm, people!"
Fights broke out at a Tesco Extra in Stretford and a woman was hurt by a falling TV.
In Salford, officers arrested a man for allegedly threatening to "smash" a staff member's face in.
Poor security numbers have been criticised by both police and shoppers alike.
The Chief Constable of Greater Manchester Police, Sir Peter Fahy, said: "The events of last night were totally predictable and I am disappointed that stores did not have sufficient security staff on duty.
"This created situations where we had to deal with crushing, disorder and disputes between customers."
South Wales Police also reported receiving a number of calls from staff at Tesco stores after they became "concerned due to the volume of people who had turned up to sale events."
There were also disturbances at a Tesco in Glasgow which had to close for a period as people clashed over discounted goods.
Many choosing to go online instead found intense traffic had overcome websites including Tesco Direct, Argos and Currys - with customers forced to wait.
Black Friday was introduced in the US on the day after the Thanksgiving holiday and was so-named because brisk business was said to have helped retailers back into profit - into the black.
UK retailers are expecting their biggest day of spending this year.
And the shopping frenzy is expected to continue right through until Monday, with estimates that £1.7bn will be spent during the so-called "cyber weekend".
More retailers have been wheeling out their best deals to lure Christmas shoppers, after Black Friday offers caused a gold rush at the tills last year.
The craze was introduced to the UK by Amazon back in 2010, with Asda joining the frenzy in 2013. This year, the supermarket is foraying into big-ticket items such as quad bikes, with some products carrying a hefty price tag of £3,500.
Bargain hunters are being reminded to "keep their cool".
John Hannett, from the shop workers' union USDAW, warned: "Black Friday can be very difficult for staff. With overcrowded shops and highly excited customers, sometimes tempers flare and things can get out of control.
"Retail staff are keen to give great customer service and are trained to provide a good shopping experience, but that can be difficult if people are angry and aggressive.
"So my message to the shopping public is clear. Enjoy Black Friday, I hope you get what you're looking for and at a great price, but please keep your cool and respect shop workers."