Speaking alongside her father, Jessica Brown, 15, said she was with friends in the Village Pizza takeaway in Common Lane when a couple came in and told them to "stand back against the wall".
"I didn't know them but they said to us to stand away so we were not hurt or seen by anyone," she said.
"I later looked out of the window and saw three people in black clothing with gas masks on and saw other people chasing them."
She said a large number of her friends were also congregated in a playground in Common Lane and many ran for cover towards the shopping precinct as the scene unfolded.
One of her friend's parents dashed to the takeaway to collect them, she said.
"When we came outside there was gas in the air, you could smell it.
"I then saw a black van with three or four people inside drive towards the car park. One of them was leaning outside the window and I later heard shots.
"It all looked like something off a film. It did not feel real at all. It was horrible."
Her father Anthony Brown told Sky News she arrived home "hysterical", adding: "She was crying her eyes out, barging through the door."
He said his first thought was to get children inside and added: "You don't want the kids outside when there's mad men running round with guns shooting."
A spokeswoman for Cheshire Constabulary said it was an "isolated" incident,
"At around 7.20pm on Saturday an incident took place near Jackson Avenue in Culcheth, Cheshire, whereby a car was stopped by armed officers from Greater Manchester Police,"
"During the contact, a male occupant of the car was shot and sustained fatal injuries.
"Two men were arrested at the scene and remain in custody. The deceased's next of kin have been informed.
"The Independent Police Complaints Commission has been informed and will independently investigate the incident with the assistance of Cheshire Constabulary and Greater Manchester Police.
"Cheshire police patrols have been increased in the area, and the community of Culcheth should be reassured that this is an isolated incident and there is no risk to the community as a whole."
An IPCC spokeswoman said: "The IPCC has been informed by Cheshire Constabulary of an incident near Jackson Avenue, Culcheth, Cheshire at approximately 7.20pm on Saturday, March 3 where a man was fatally shot by armed officers from Greater Manchester Police.
"The incident has been referred to the IPCC who will independently investigate. IPCC investigators are at the scene and working with officers from both Greater Manchester Police and Cheshire Constabulary."