At least six Brits were arrested in Madrid before Saturday’s Champions League final, according to police, including one man accused of performing a sex act in public.
The 33-year-old is said to have climbed into an elevated position in Puerto del Sol - one of the focus points for Tottenham and Liverpool fans this week - stripped off, then committed the offence.
He was also later arrested for assaulting an Italian woman, with footage shared on social media suggesting the sex act was performed in front of a large crowd late in the evening.
Police have declined to release details about the suspects or their current custody status, despite requests for them to do so.
It is believed the man was among four Britons arrested on Friday for a string of offences, including attacking police.
Two further fans were arrested around the Wanda Metropolitano Stadium in the hours leading up to Saturday's final.
One was detained for possessing narcotics and assaulting police, the other for striking an official so badly that they required medical attention.
Police had warned football fans they would take a "robust" attitude to bad behaviour, and formed a ring of steel at stadium checkpoints as scores of supporters tried their luck by accessing the game without genuine tickets.
There were ugly scenes around three hours before kick-off when one young woman from North West England was dragged back to the security holding pen after she fled to the nearby metro station when her ticket was found to be a fake.
