Tripoli - Libyan state prosecutors are investigating unidentified guards purportedly shown in a video beating a son of Muammar Gaddafi held in a Tripoli prison, a statement said on Monday.
The video was posted a week after a Tripoli court sentenced Gaddafi's son Saif al-Islam and eight former regime officials to death over crimes during the 2011 uprising that toppled his father, who was later captured and killed by rebels.
Rights groups said the trial was riddled with legal flaws.
Libya is in chaos with two governments fighting each other with the help of former anti-Gaddafi rebels, while Islamic State militants have gained a toehold in a growing security vacuum.
The video appears to show guards teasing and beating Saadi Gaddafi, who was extradited to Libya by Niger last year. He has been held since then in a Tripoli prison facing charges over the killing of a soccer player when he headed the Libyan Football Federation, as well as other crimes.
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In the video, Saadi himself is beaten on his face while being questioned and later on his soles tied to a rack. He is also forced to listen to other screaming prisoners being beaten outside the room he was held in.
The video's authenticity could be not be independently verified.
After escaping Libya in 2011, Saadi was held under house arrest in the Niger capital Niamey before being extradited.
Tripoli's prosecutor general said in a statement it had launched an investigation to identify the guards in the video and "to take the necessary legal action against them".
Saadi had a brief career as soccer player in Italy and often lived the playboy life during his father's longtime rule.
Last week's court ruling against Saif al-Islam was passed in absentia since he has been held since 2011 by a former rebel group in Zintan, a western region beyond Tripoli's control.
Other former Gaddafi regime officials sentenced to die by firing squad included former intelligence chief Abdullah al-Senussi and ex-prime minister Baghdadi al-Mahmoudi.