Police are investigating a fight on a bridge over the Chichester bypass on Monday afternoon.
A 15-year-old boy has been arrested on suspicion of assault.
A 54-year-old man was injured and taken to hospital.
Officers are urging the public not to share videos of the incident online, warning that doing so could affect the ongoing police investigation.
A statement on Chichester Police Facebook account says: "We are aware of footage on social media of an altercation involving a teenager in Chichester.
The incident happened on a bridge over Chichester bypass at about 2.45pm on March 10.
A 15-year-old boy was arrested on suspicion of assault. He has been bailed with strict conditions, pending further enquiries.
The victim, a local man aged 54, sustained injuries and attended hospital. He will continue to be supported by officers.
Officers have asked the public not to speculate about, comment on, or share footage of the incident, as this will impact the ongoing, active investigation.
Anyone with information about the incident can report it to us online, quoting serial 817 of 10/03."
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