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Last Saturday night, a group of hooded men emerged from a small supermarket on Barcelona's Carrer Aragó, and hurriedly got into the Spanish National Police vans which were parked outside. With them was one handcuffed man.

The moment when they left the grocery store was caught on video and it has virtually gone viral. But what's behind these images?

The Spanish police have admitted to that they deploy plainclothes or undercover officers among the protesters. They are Spanish police officers who infiltrate protests and their official mission is to detect people who cause unrest and stop them. They dress appropriately for the occasion. Thus, these days in Barcelona they've dressed to merge into the protest crowd, wearing hooded sweaters.

On Saturday night, during the incidents in downtown Barcelona, ​​a group of these plainclothes officers was pursuing one of the protesters who, according to police sources, "was acting violently." He was arrested at the door of a supermarket on Carrer Aragó. It was at that point that they noticed that a CDR group was approaching. "They were warned by WhatsApp," say the same police sources. And that was when they took refuge in the supermarket.

Inside, with the arrested person handcuffed, they told the shop assistants to pull down the shop's roller door and they called for police reinforcements. After a few minutes, several vans arrived to pick up the arrested person and the undercover agents themselves. That's the moment which is seen in the images, which set off everybody's alarms about the presence of police infiltrators in the protest crowd.

The arrested protester was taken directly to the police station and then later appeared before a judge. As part of the requirements of the arrest process, in order to explain to the judge what they were doing with the arrested person in a supermarket - to prove that they stopped him on the street and subsequently entered the shop - the police asked for images from the supermarket security cameras. But the interior cameras did not record the scene. Or at least that's what the owners of the establishment told the police. Sources in the Spanish National police say that asking for such images is standard procedure in this type of case to prove what they did with an arrestee from the moment of being placed under arrest.