Media playback is unsupported on your device Media caption Brian Spain filmed shoppers fighting over merchandise at a Walmart in Elkin, North Carolina

Several outbreaks of violence have marred the US Black Friday shopping frenzy, as bargain-hunters besieged malls across the US.

In Chicago police shot an alleged shoplifter; a robber shot a shopper in Las Vegas; and a California police officer was injured in a fight.

Black Friday, the day following the Thanksgiving holiday, is the biggest shopping day of the year in the US.

This year it began even earlier amid a trend for Thanksgiving openings.

Twelve national chains opened their doors on Thursday, advertising aggressive discounts.

Pepper spray

Some 15,000 shoppers stormed the flagship Macy's in New York City as it opened for the first time ever on Thanksgiving evening.

Pointing at the mobbed department store, Brazilian tourist Luis Figueiro told Reuters news agency: "This is madness.

"There are so many people here, you can't see any of the things on sale."

There were several incidents of retail-related disorder across the US:

In Chicago, a police officer shot a suspected shoplifter driving a car that was dragging a fellow officer at a Kohl's department store. The suspect and the dragged officer were treated in hospital for shoulder injuries. Three people were arrested, reports the Chicago Tribune

One woman used a stun gun on another as a fight broke out at a shopping centre in Philadelphia, reports a local NBC station

A shopper in Las Vegas who was carrying a big-screen TV home from a Target store on Thanksgiving was shot in the leg as he tried to wrestle the item back from a robber who had just stolen it from him at gunpoint, reports the Las Vegas Sun

At a southern California Walmart store, a police officer's wrist was broken as he tried to break up a fight between two men in the queue outside; there were two more fights over goods inside, reports the San Bernadino Sun

A 23-year-old man was doused with pepper spray and arrested after he allegedly attacked a police officer responding to an argument over a television at a Walmart in Garfield, New Jersey, reports the Star-Ledger

Despite Walmart's pledge to overhaul its crowd-control measures, scenes of mayhem such as this one were apparently filmed at a store in Fort Worth, Texas

Two arrests were made after a man was stabbed in an argument over a parking space at a Walmart in Virginia, reports local television station WVVA

Workers' groups have protested that the trend towards Thursday opening means retail employees can no longer spend the day at home with their families, which is supposed to be the point of Thanksgiving.

Protests

Some retail analysts have begun to dub the holiday Black Thanksgiving, or Grey Thursday.

Workers held demonstrations on Friday outside Walmart stores in the city of Ontario, California, and in Elgin, Illinois, demanding better pay and conditions.

There was anecdotal evidence that the Thursday openings have led to an easing off in consumer footfall on Black Friday itself, though the increased popularity of online shopping could be another factor.

By late Friday morning, the number of shoppers in many stores was more typical of a normal Saturday than the usual frenetic start to the holiday season.

Downtown Manhattan, for example, was busy, but not at saturation level.

The US celebration of Thanksgiving is always marked on the fourth Thursday in November.

The day after is known as Black Friday because that was the time of year when retailers began making a profit, or moved out of the red and into the black.

Some 97 million Americans hit the shops on Black Friday, according to the National Retail Federation.

Last year on the day Americans spent $11.2bn (£6.8bn).

Image caption Macy's in New York City opened its doors at 20:00 on Thanksgiving day

Image caption These two brothers were comparing iPad prices in Dunwoody, Georgia

Image caption There were queues outside Toys R Us in Times Square in New York

Image caption There were protests at Walmart stores - such as this one in Elgin, Illinois - by workers demanding better pay and conditions

Image caption These happy consumers leave Walmart in Troy, Michigan

Image caption Modern Thanksgiving celebrations: Buying two TVs at Target in Colma, California