The 36-year-old woman said she was shopping in the Morrison’s on Wood Green High Road when she noticed someone close behind her.
Upon turning around she allegedly saw the man, described as white and aged in his late 20s, kneeling down and holding his phone under her skirt.
The horrified woman challenged him, but he just replied “no English” and fled the store.
Haringey police have released CCTV footage from outside the supermarket - which they believe shows the man running away – in a bid to encourage any witnesses to come forward.
Officers said the alleged incident happened on Sunday March 12 at 3.25pm and described the suspect as having short dark hair, a beard and wearing a dark hooded coat.
Detective constable Annette Corry said: "We are keen to identify the man in the CCTV footage so we can speak to him about the incident. We would also urge anyone who witnessed this incident to contact us."
Anyone with information is asked to call Haringey CID on 020 3276 3153, tweet @MetCC or call Crimestoppers anonymously on 0800 555 111.
