Police were called to an alleged theft at Barclays Bank in Tooting and while they were speaking to the suspect he collapsed.
A witness reported seeing a man on the floor being detained by police and a queue of people.
MPS Wandsworth tweeted after the incident on Saturday afternoon and said: "Police were called to an allegation of theft. While police were speaking to the suspect, he collapsed.
"London Ambulance Service were called. He suffered an epileptic fit, our officers provided care until London Ambulance Service arrived.
"The initial allegation of theft transpired to be a misunderstanding. We understand the male has fully recovered."
Did you see what happened? Email ssleigh@london.newsquest.co.uk
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