PC Swaleh Chaudhry, 36, was caught out when the victim spotted his phone being used to spy on her at the store in the Southside Shopping Centre in Wandsworth.
After he was arrested, in his home officers found a cache of images of children being sexually abused, with infants said to be among the victims.
Chaudhry appeared at Wimbledon magistrates court on Friday, pleading guilty to three charges of making indecent images of children, possession of extreme pornography involving animals, and voyeurism.
Deputy Chief Magistrate Tan Ikram remanded the officer, who is attached to the Met Police’s taskforce, into custody until sentencing at Kingston crown court at a later date.
Scotland Yard said Chaudhry was off-duty at the time of the voyeurism incident, on March 30, and has been suspended.
“A female complainant was in the fitting room trying on clothing and she stated she heard rustling in the fitting room next to hers”, said prosecutor Suleman Hussain.
“Under the benches was a gap in between the fitting rooms, she saw a black mobile phone with the camera facing her.
“She saw the camera appearing to follow her as she was moving, she began to scream ‘are you recording me?’.
“There was a response from the cubicle next to hers, saying no. She came out and warned her boyfriend, and the defendant was then challenged.”
The court heard Chaudhry was detained by security until police arrived to arrest him.
Mr Hussein said electronic devices at the PC’s Wandsworth home were investigated, uncovering more than 1000 images of children being sexually abused including at least 50 in the most serious category.
He told the court some of the pictures are “images of infants”, while he also had videos of himself performing sex acts.
The court heard Chaudry says the images were downloaded in 2012 and 2014, and he wants to seek treatment. The Met’s Directorate of Professional Standards are aware of the case, and a referral has been made to the Independent Office for Police Conduct.