Police have been called to supermarkets across the UK amid crowd surges as people hunt for "Black Friday" offers.
Greater Manchester Police appealed for calm after attending seven Tesco shops, at which three men were arrested and a woman was hit by a falling television.
The force said the issues were "totally predictable" and it was "disappointed" by shop security. Tesco said only a "small number" of stores were affected.
Police were called in places including Dundee, Glasgow, Cardiff and London.
Originating in the US, Black Friday is becoming a major UK shopping day.
Visa has predicted that UK shoppers will spend £518m online on cards on Friday - which would make it the country's biggest internet shopping day in history.
Police in Manchester tweeted: "At least two people arrested at #BlackFriday sales events already this morning. Keep calm people!"
The force said there had been "disturbances" at the seven Tesco stores.
Incidents at those stores and other locations included:
About 200 shoppers refused to leave a store in Middleton "despite being told stock had all gone"
Fights broke out between shoppers in Stretford, and a woman suffered "minor injuries" after being hit by a falling television. The store was closed at 00:36 GMT
A man was arrested in Salford after he threatened to "smash a staff member's face in"
A woman broke her wrist in a crush, with Greater Manchester Police's deputy chief constable Ian Hopkins describing shoppers' behaviour as "appalling"
In Wigan, officers were called to reports of "several hundred people trying to enter the store". Police added: "Two men were ejected before control was regained"
There were reports of fighting in a store in Hattersley, where a man was arrested for a public order offence
A man was arrested for assault at a store in Green End
Bicester Village shopping outlet in Oxfordshire had to temporarily close after roads around it became gridlocked by Black Friday shoppers.
