Two men were arrested on suspicion of grievous bodily harm (GBH) at a Tommy Robinson march in central London.
The Metropolitan Police said they attended reports that a participant of the Stand Up To Racism counter-protest was assaulted, adding the victim suffered a head injury.
The force wrote on X: “Two men have been arrested on suspicion of GBH-level assault following an incident in Victoria Embankment Gardens where a participant in the Stand Up To Racism protest was assaulted.
“The victim sustained a head injury. He was given first aid by officers and will be taken to hospital to be checked.
“We are aware of some suggestions on social media that the arrests were related to the carrying of a flag which is not the case.”
Around 1,000 police officers were mobilised to “keep the peace” in the capital on Saturday as three marches took place simultaneously, including the Stand Up To Racism event, Mr Robinson’s demonstration entitled “Uniting The Kingdom” and a Trans Pride protest.
The police had already used powers under the Public Order Act to try to keep the opposing groups apart amid fears of serious disruption.
Chief superintendent Colin Wingrove, who was in charge of this weekend’s operation, warned at the start of the day that police would “intervene decisively” to deal with protesters who break the strict conditions about when and where they are allowed to demonstrate.
Thousands of demonstrators, in an event organised by far-right activist and former English Defence League leader Mr Robinson, whose real name is Stephen Yaxley Lennon, marched from the Royal Courts of Justice to Trafalgar Square, setting off at 12.45pm.
In what was advertised as the “biggest patriotic rally the UK has ever seen”, the demonstrators were heard chanting “England” and “Rule Britannia”, as well as Mr Robinson’s name. Israeli flags were also seen being waved at the event.
A man walked past the protest at the square holding a Palestinian flag and attendees could be heard booing. Some attempted to chase the person as he was escorted by a group of Met officers down Cockspur Street.
Just a few hundred metres away, Stand Up To Racism demonstrators marched from Russell Square to Whitehall, holding up placards calling for the end of British weapons supplies to Israel.
The counter-protest, which set off at 1.30pm, was run with the support of the Trade Union Congress (TUC) and 11 other trade unions, as well as Jeremy Corbyn’s Stop the War Coalition, Peace & Justice Project and Unite Against Fascism.
A spokesperson for the Met Police said details of the incident were still being collected and that a comprehensive account would be released later on Saturday.
Tommy Robinson, who caused controversy running counter-protests to the pro-Palestine marches during Armistice Day last November, was pictured addressing crowds at Trafalgar Square.
Actor turned political activist Lawrence Fox was also seen among the crowds at the event.