KIEV, Ukraine — As opposition leaders negotiated with President Viktor F. Yanukovich to defuse Ukraine’s violent civil uprising, new evidence emerged of brutality by the authorities, including a video of a protester stripped naked except for boots by a group of officers from the feared Berkut riot police.

The video shows the naked man standing on snow-covered streets, being photographed by one police officer while several others looked on. Another officer is seen grabbing the man by the back of the neck, forcing him to hold an ice scraper, then slapping him on the head and kicking him as he is directed into a police bus. Welts are visible on the man’s back as he climbs into the bus.

The Interior Ministry, which oversees the riot police, issued an apology and said the episode was under investigation.

The video stood to further inflame demonstrators who were still reeling from the first violent deaths in the two-month-long uprising, threatening to upend a fragile cease-fire. It seemed to reinforce evidence that the authorities or their surrogates were engaging in other brutal tactics, including the killings of protesters.