Saleem Patel also clicked images of women using the toilet and breastfeeding, a court was told.
He was caught when two off-duty police officers spotted him photographing a mother nursing her child in a park.
A search of Mr Patel's phone and computer found over 9,000 images he'd secretly made of women, Hull Daily Mail reported.
He told police officers he regularly travelled about from home in Paragon Street to take pictures of women on the underground.
Mr Patel pleaded guilty at Hammersmith Magistrates Court yesterday to one count of outraging public decency relating to snaps taken between January 1, 2013 and April 16, 2015.
Prosecutor Tom Gill said: "In relation to the facts of this case, on April 16 this year two off duty police officers where in Covent Garden when they noticed the defendant taking photos of a mother breastfeeding."
"The officers called Covent Garden Security and the police were called. Adjourning sentence until September 9 to allow time for pre-sentence reports to be prepared, Deputy District Judge Susan Cooper said Patel could be facing jail," Gill said.
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