Police are hunting pubgoers who might have filmed a fight connected to March for England on Sunday.
It took place in the North Laine area of Brighton.
Shortly after 2pm two groups of people were involved in serious disorder between The Dorset pub and Infinity Foods at the junction of North Street and Gardner Street.
A Sussex Police statement said: "This serious incident is believed to have been in connection with the march and counter protest which had taken place on the seafront earlier on in the day."
Detective Chief Inspector Carwyn Hughes said: "We have already made 27 arrests connected to the disorder in the city and our attention is now focused on this particularly nasty incident.
"We already know that video footage of the incident was taken by someone in The Dorset pub and would appeal to this person to come forward to the police, as we would to anyone else who witnessed it.
"We will not tolerate this sort of behaviour and will be making every effort to find those people involved and bring them to justice."
The march and counter protest were largely peaceful, however there were small incidents of disorder elsewhere in the city.
Superintendent Steve Whitton, who led the operation, said: "Our investigations team are now looking into the incidents of disorder around the city to bring people responsible to justice.
"We have had no reports of anyone seriously injured, but I know there was disruption to the traders in the city.
"Officers will be visiting premises we know were affected on Sunday throughout the city.
"I would like to thank residents, traders and visitors for their patience and co-operation. It is now very important we work in partnership with the businesses and city council to look at the impact it has for any future events."
If anyone has any details on the fight please contact police on 101 or email firstname.lastname@example.org quoting reference 474 of 27/4.