At least one person has been injured as a chain store's sales promotion descended into "bedlam".
Asda stores across Northern Ireland were scenes of chaos, according to callers to the BBC.
Several customers said they left empty-handed after finding stores had rapidly emptied of stock advertised in Asda's 'Black Friday' promotion.
Asda said it "had full security teams in stores and extra colleagues to assist customers in the aisles".
The company said reaction to the promotion, on what is said to be the busiest shopping day of the year, had been "phenomenal".
In an Asda store at the Westwood centre in west Belfast, a woman was taken to hospital with a suspected broken wrist after scenes that a customer described as "bedlam".
Hundreds of people queued outside the shop from 05:00 GMT for cut-price goods that included televisions sets which were reduced from £179 to £99.
However, when it got under way three hours later, there was what one woman described as a "stampede" with people wrestling over the goods.
There were 24 sets on sale at the reduced price.
Callers to Radio Ulster's BBC Talkback programme reported similar scenes at Asda branches across Northern Ireland as shoppers jostled for bargains.
A grandmother who visited the store's Portadown branch in County Armagh with her grandchild said she had feared for the child's safety and described the scene as "mental".
Another man, from County Antrim, said: "They were not doing it (the promotion). There were no offers at the Larne store."
An Asda spokesperson said: "This is the first time Black Friday has been done on this scale in stores across the UK and our customers were eager to take advantage of the great offers available to them.
"We planned for high demand and the half-million Black Friday products on offer to our customers have been selling quickly since 8am.
"Throughout the event the safety of our customers is of vital importance, and to ensure our stores can cope with the extra footfall, we have full security teams in our stores and extra colleagues to help assist customers in the aisles."
Black Friday refers to the fourth Friday before Christmas, claimed to be one of the busiest shopping days of the year.