Footage has emerged showing the moment the violence happened in between cars outside the shop on Penn Road in Wolverhampton.
Steven Gill saw the mass fight unfold at around 8pm on Thursday after visiting the shop to get some beer after leaving The Rose and Crown pub nearby.
Mr Gill, 43, saw one woman get knocked out on the ground after being punched by a man, while several other people had blood on their faces and clothes.
Footage he took from the incident shows people gathering in the store's car park - many of whom were smartly dressed as if attending a function - blocking the way for cars who have to beep and try to get past.
Mr Gill said: "I was picking some beer up [when this happened] and it seemed to me like they were from a wedding, or a function of some kind.
"There were people fighting with one another and there was a one who had been knocked out on the floor and people with blood on their faces.
"I didn't hang around after that. It was quite shocking because I've lived in this area for 30-odd years and I'm always walking up and down here, and there's children who walk this way, and I've never seen anything like this happen before. It's a nice and quiet area – it's shocking to see."
Several young children were nearby as the punches were thrown.
The emergency services were called but when paramedics arrived no patients were found.
A spokesman for the West Midlands Ambulance Service said: "We were called at 8:46pm [on Thursday] to reports of an assault in Penn Road, Wolverhampton.
"We sent one ambulance and a paramedic officer. However, upon arrival, no patients relating to an assault were found."
West Midlands Police has been contacted for comment.
The Express & Star has chosen not to publish the video due to the distressing nature of the violence and to avoid identifying the children involved.