Today is Black Friday - one of the biggest shopping days of the year.

Police were called to at least four supermarkets in London alone because of over-crowding, while shoppers took to Twitter to describe "carnage" in stores, with some reports of people being stripped of cheap TVs and other goods by rivals in the scramble.

Many choosing to go online instead found intense traffic had overcome websites.

Black Friday was introduced in the US on the day after the Thanksgiving holiday and was so-named because brisk business helped retailers back into profit - into the black.

It is now popular across Europe and includes Cyber Monday.

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.

Sales began at midnight with hoards of bargain-hunters descending on stores in a rush to pick up goods ahead of Christmas.

Shoppers are queueing waiting for doors to open as stores prepare to offer huge discounts.

Irish people spent €39m on Cyber Monday last year.

Lorna Barker is an online shopping expert with PriceSpy.ie. She told Newstalk Breakfast that Irish shoppers are taking to this trend with a vengeance.

Grace Duffy is from the European Consumer Centre and has this advice for shoppers.

In a Walmart in California, a woman pepper sprayed other shoppers, including children, for an "XBox Special" on Black Friday in 2011.

Twenty people were treated for minor injuries, and according to CNN, the woman responsible was able to pay for her purchases and leave before police arrived, but later turned herself in. There is even a video of the whole incident: