WARNING: GRAPHIC CONTENT
*Couple were spotted having sex on a bench at the busy Liceu Metro station
*A local filmed the pair half-naked in front of commuters waiting for a train
*Video shows a group of people watching X-rated episode in utter disbelief
*Tube bosses were not aware of incident but said they would investigate.
*This is the shocking moment a couple were caught having sex on the platform of a busy tube station in clear view of stunned commuters.
The unbelievable scene was filmed by a local who spotted the amorous pair on a bench at Liceu Metro station in Barcelona, Spain.
The footage was captured in the early hours of Sunday morning and showed the couple half-naked on the platform while other passengers waited for a train to arrive.
A local man is seen walking past the couple and choosing not to look over before a group of young people enter the station.
They spot the X-rated episode and can be heard laughing while looking on in utter disbelief.
The incident took place in the centre of the old part of the city known as the Gothic Quarter - an area popular with tourists on a stag do or a hen weekend.
A friend of the video maker told Catalan daily El Periodico: 'We are fed up with these sorts of situations and that's why we've decided to register a formal protest and go public with it.'
Local news also reported that it was unclear whether the pair let their emotions get the better of them after too much drink or whether they were partaking in a dare.
Tube bosses said they were not aware of the incident but would be investigating.
The footage emerges following a similar episode, which occurred in August, when a tourist filmed a couple having sex in the Costa Blanca resort of Torrevieja.
Shocked holidaymakers called police to interrupt the couple, but they reportedly told officers to 'go away' when they arrived on the scene.
The video went viral after it was shared on social networking sites and shown on regional TV.