Bargain hunters have brawled in stores and even attempted to pose as staff to beat the lines as Black Friday sales madness takes hold in the US.

Police were called to the Riverchase Galleria shopping centre in Hoover, Alabama to break up a fight between at least two women, which forced the shopping centre to be shut early.

A video of the incident posted online showed a security guard holding one woman down on the floor while two women continued to punch and kick each other, with tables of clothing being pushed over and a crowd of onlookers watching the chaos.

A witness told a local Fox 6 TV reporter that a shoe was thrown during the melee, flying over a railing and hitting a baby.

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Reports of gunfire spread on social media, but authorities said they were not confirmed and that no one was shot.

A 19-year-old man sustained life-threatening injuries when he was shot in a shopping centre car park in Columbia, Missouri on Thursday night, however local authorities said it appeared to be an accident and not related to Black Friday shopping.

Police said two men who knew each other were sitting in a vehicle when one fired a shot as he placed his gun in his holster, KSHB Kansas City reported.

Elsewhere, some went to extreme lengths to bag a bargain by attempting to pose as store staff to get inside first.

A Twitter user wrote it was a "good thing [he] kept [his] vest from Walmart when [he] quit" and that he would be "skipping everybody" by wearing it to pose as a staffer.

Americans are predicted to spend $US20 billion ($29b) during the Thanksgiving weekend, according to the National Retail Federation.

A survey found 115 million Americans planned to hit the shops on Black Friday alone.