This is the terrifying moment rival groups of youths clashed in Blackpool Road in Preston.
The video, which has been shared more than 2,000 shares online, shows a youth standing on a car in the middle of Blackpool Road holding up motorists.
The footage, which was filmed on a phone by a driver caught up in the incident, also appears to show one young man with what looks like a metal bar in his hand, which he then appears to hit another person with.
Shortly after the young man is joined by others on bikes, squaring up to each other aggressively.
A Lancashire Police spokesman has confirmed an investigation was under way.