Sussex Police said its officers were called to the shop in East Street, Chichester, shortly after 3.30pm on Wednesday to reports that a group of teenagers had become involved in an altercation. A shop worker also reported being assaulted.
The force said two children were arrested in connection with the incident.
Ms Buchanan told The Mirror newspaper that the rangers “started to get aggressive towards them and one of them got into my son’s personal space and he asked: ‘Can you get out of my face?’
“What I have been told is the ranger pushed him and my son pushed back.
“He was then slam-dunked to the floor and [they] sat on top of him as you see in the video and they put plastic handcuffs on him – the police then arrived and arrested my son.”
Speaking on Times Radio, Ms Buchanan said her son was followed by the rangers into the shop and added: “When he got to the shampoo counter he was joking around about being followed by these two guys whereupon they snapped, grabbed his arm, threw him on to the ground, sat on top of him.
“They got plastic handcuffs, handcuffed him and yanked his arms up behind his back whereupon my son used some understandably unparliamentary language but also screamed: ‘It hurts, it hurts, get off me.’
“A friend of his came to his aid and was also subsequently arrested for assault, which was committed in prime to try to get these two men off my child.”
She said a member of the staff reported being hurt in the incident, but said this happened after her son was detained by the rangers.
She added: “As far as I understand, he was arrested for assault, and I promise you I am not joking, while being pinned to the floor he threw his head back and he has been accused by one of these security guards of a backwards headbutt.”
Ms Buchanan’s son is now at home, having been released by the police in the early hours. She added: “He is relatively calm because he is quite content when the footage is reviewed, the charges will be dropped.
“There needs to be an investigation into the police handling of this. For an hour and a half, I didn’t even know where my son was or had been taken.
“I also think there needs to be a thorough review of the use of these security guards on our streets because they are on our streets all across the country, they are completely unregulated. What is the process of due diligence by which they are hired? What is the training procedure? What is their remit?”