Police were called to stores in London and other British cities in the early hours as chaos ensued at supermarkets and large stores holding Black Friday sales.

The consumerist frenzy that follows Thanksgiving is the biggest shopping day of the year in the US and is rapidly growing in popularity in Britain.

Police were called to at least four supermarkets in London due to overcrowding and shoppers fell to the ground as they rushed into an Asda store in Wembley.

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Members of staff try to control the crowd clamouring for flat screen TVs at an Asda superstore in Wembley, northwest LondonLuke MacGregor/Reuters
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...And they're off...Luke MacGregor/Reuters
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...Several people compete for one flat screen TV at the Asda store...Luke MacGregor/Reuters
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..."It's mine!" "I saw it first!"...Luke MacGregor/Reuters
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...Some bargains are clearly worth hanging on toLuke MacGregor/Reuters

Across the pond, millions of Americans headed to the stores for bargains in what's quickly becoming a new tradition on Thanksgiving, a day once dedicated solely to family and feasting. Just a few years ago when a few stores started opening late on the holiday, the move was met with resistance from workers and shoppers who believed the day should be sacred.

Last year, more than dozen major retailers opened at some point on Thanksgiving evening. And this year, at least half of them — including Target, Macy's, Staples and JC Penney — are opening earlier in the evening on the holiday.

The Thanksgiving openings are one way retailers are trying to compete for Americans' holiday dollars. Previously, stores would advertise bargains on the day after Thanksgiving, but increasingly, they've been pushing those promotions into the holiday itself, to grab deal-hungry shoppers' attention.

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Children wait for the Toys R Us store in Times Square, New York, to openAndrew Burton/Getty Images
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The doors open at a JCPenney store at the Newport Mall in Jersey CityKena Betancur/Getty Images
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Shoppers wait to enter Macy's in New York to kick off Black Friday salesAndrew Kelly/Reuters
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Protesters hold signs aloft outside Macy's in New York before the start of Black Friday salesAndrew Kelly/Reuters
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A man squeezes his newly purchased 55" screen TV into his car outside a Best Buy in Fairfax, Virginia, during the Black Friday salePaul J Richards/AFP
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People reach out for discount coupons at a JCPenney store in Jersey City, New JerseyKena Betancur/Getty Images
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People load their newly purchased 65" TV into their van outside the Best Buy in Fairfax, VirginiaPaul J Richards/AFP

Not every store opened on Thanksgiving, though. Some, including GameStop, Costco and Ikea, said they won't open because they want their workers to enjoy the holiday with their families.