"Black Friday" descended into chaos today as fights broke out at stores and websites of leading chains buckled under the strain.
Police were called to several supermarkets around the country overnight as thousands of customers sought bargains on what retailers hope will be one of the busiest shopping days of the year.
At least two people were arrested and a woman was hurt after being hit by a falling television, while officers were forced to shut some outlets to restore order.
Those trying to take advantage of discounts online also ran into problems, with shoppers forced to wait to access the websites of Tesco Direct, Currys and Argos.
Police attended shops after staff, and some members of the public, raised concerns over the volume of people attending the Black Friday sales.
A number of shops opened at midnight, boasting massive discounts on a range of goods, but shoppers described scenes of "carnage" as people scrambled for the best deals.
Greater Manchester Police (GMP) said two people were arrested at Black Friday sales events.
The force said on its Twitter feed: "Keep calm, people!"
Officers were called to seven Tesco stores across the area.
Shortly before midnight they were called to Tesco Central Park in Wigan after reports of several hundred people trying to enter the store. Two men were ejected before control was regained.
The Tesco at Ellesmere Shopping Centre in Walkden was closed after a crowd of more than 500 people attended.
The Tesco Extra in Stretford was closed after fights broke out between shoppers trying to get their hands on sale stock.
A GMP spokesman said: "A woman suffered minor injuries after being hit by a falling television and an ambulance was called. The store was closed at 12.36am, just over half an hour after the sales event started."
A man was arrested on suspicion of assault after he was challenged by staff at the Tesco on Woodrow Way in Salford. He had threatened to "smash" a store employee's face in, police said.
Officers were called to Tesco Extra on Barton Road, Middleton, following reports that around 200 people would not leave, despite being told stock had all gone and the doors were locked.
A man was arrested for a public order offence after reports of fighting after around 300 people descended on the Tesco Extra on Stockport Road, Hattersley.
Police were called to four supermarkets in London as people gathered at Tesco stores in Edmonton, Willesden and Surrey Quays and an Asda in Capitol Way, Edgware.
Images posted online showed chaotic scenes in Edmonton, with witnesses reporting "mayhem" as discounted coffee machines went on sale.
The area has seen disturbances before, with several people hurt as crowds of thousands forced a nearby Ikea to shut on its opening night in 2005.
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".
Officers attended a store on Excelsior Road in Cardiff twice after midnight over concerns about "customer conduct". police said.
No arrests were made and police said they had advised management at the stores to prevent further issues.
One person who bagged a coffee machine at a knock-down price tweeted: "After the absolute carnage that is Black Friday in a Tesco Extra in the North of England, I walked away with a £100 coffee machine for £30."
A message on the Tesco Direct homepage said: "Sorry for making you wait
"Lots of people are shopping with us at the moment and we need to ask you wait briefly. Don't worry, we'll redirect you to the website as quickly as we can."
Those trying to shop online with Currys were told they would have to wait 25 minutes for access. "We're really sorry that the huge demand for our deals means you're having to wait," the website said this morning.
"We really appreciate your patience and we'll have you on our site as soon as possible."
A message on the Argos homepage said it was limiting access because of "very high volume" and urged visitors to try again in a few minutes.
There were also disturbances in Scotland. Police were called to Tesco at Silverburn in Glasgow which was closed for a period as people clashed over discounted goods.
Sarah Coubrough wrote on Twitter: "Silverburn shut cos shoppers going mental with the sales. Think I'll stick to the online shopping today."
Emma Somers posted: "Police called to the Tesco at Silverburn cause folk are fighting over the bargains for Black Friday."
At another Tesco store in Dundee, police were called to similar scenes and a decision was taken by staff to shut the shop.
A statement from Police Scotland's Tayside division said: "For information the Tesco Extra, Kingsway West, Dundee has been closed at this time due to excessive numbers attending to purchase sale items.
"This decision was made by Tesco staff to ensure the safety of all their customers."