NOT JUST ONE DAY

This year, some retailers -- including Amazon, Argos, Marks & Spencer and Dixons Carphone -- have stretched "Black Friday" discounts over several days.

Amazon is offering more than 7,000 deals over the course of a week, while Dixons Carphone is offering over 3,000 deals over 10 days.

Researcher FootFall estimates Black Friday in-store shopper numbers will be down by up to 4 percent from 2014.

Dixons' Currys PC World chain reported unprecedented online demand online, with a 53 percent increase in hourly web traffic compared to last year.

Social media indicated stores across the country were quieter than last year and shoppers better behaved.

At Tesco, Britain's biggest retailer, Black Friday started on its website at midnight. Doors opened at 250 of its largest stores earlier than normal at 0500 GMT.

"We're really happy with how it's going so far. What you haven't seen is the big crazy crushes," said a Tesco spokesman.

"It's going really well online," he added.

At the huge Westfield shopping centre in east London, close to the London 2012 Olympic stadium, trading was brisk rather than frenzied.

Shaquille Simon, a 21 year-old McDonald's manager, said he bought two televisions online from Tesco at midnight, before heading for Westfield early Friday.

"If I see something I like I'll buy it. Simple as that," he said. He noted the mall was less crowded than he expected.

Asda, which is owned by Wal-Mart and was one of the UK's Black Friday pioneers, stepped back from the event this year, blaming "shopper fatigue".

"Asda's retreat might encourage other retailers, who have already planned for and bought for 2015, to scale back on Black Friday in 2016," said Bryan Roberts of researcher Kantar Retail.