Police have been called to more than 10 UK supermarkets amid crowd surges as people hunt for "Black Friday" offers.
Greater Manchester Police appealed for calm after attending five Tesco stores, making "at least two" arrests, and a Trafford store was closed shortly after the sale began at 00:01 GMT on Friday.
Police went to three Tescos and an Asda in London, and South Wales Police were called to a number of Tesco stores.
Originating in the US, Black Friday is becoming a major UK shopping day.
Police in Manchester tweeted: "At least two people arrested at #BlackFriday sales events already this morning. Keep calm people!"
In London, police were called by both Tesco staff and queuing shoppers as people gathered at the Glover Drive store in Edmonton.
Officers also attended Tesco stores in Willesden and Surrey Quays and an Asda in Edgware at around midnight.
The Metropolitan Police said officers worked with staff to "ensure that sale-goers were able to enter and exit the stores safely", and said no-one had been injured "as a result of the overcrowding".
In South Wales, police said they were called by concerned staff at a number of Tesco stores.
A spokesman said officers were twice called to Tesco in Excelsior Road, Cardiff, just after midnight, when concerns were raised about customer conduct inside the shop. No arrests were made.
Black Friday - historically the big sales day in America that follows the Thanksgiving holiday - has been adopted by an increasing number of shops in the UK.
It was brought over by online store Amazon four years ago after internet shoppers noticed the US got the best deals.
Tesco has previously said it expects Black Friday to beat Boxing Day sales in 2014.
Its 24-hour stores began selling a minute after midnight, and extra staff have been laid on in some areas.
Culled from BBC News.