A member of Ukraine's elite Berkut riot police assaulted and humiliated a naked protester in freezing cold temperatures in Kiev, according to a video posted on a newspaper website.
The Ukrainian interior ministry swiftly apologised for the incident.
The man had apparently been arrested during the clashes between police and protesters that have rocked the Ukranian capital over the past few days.
The video, which was posted on YouTube and rapidly broadcast by Ukrainian opposition channels, shows the arrested protester naked except for his socks and being roughly pushed into a police bus.
But he is then called back and is forced to stand still on the snow in the freezing temperatures while a police officer in a balaclava films him with his mobile phone.
The man is then forced to pose for pictures clutching a spade, alongside another balaclava-clad officer who mockingly puts his arm around the prisoner. This officer then hits him roughly on the back of the head.
The officer then leads the prisoner back to the van, giving him a final kick.
The interior ministry said it was apologising for the "unacceptable actions of the people in police uniform".
It said an internal investigation had been launched into the circumstances of the incident.
Interior Minister Vitaliy Zakharchenko reminded his staff of the "inadmissibility of such actions and" and called on police to "stick to the norms of the law and morals".
The head of the protest camp said the victim had been identified and was being cared for.