Images captured by a Reuters photographer showed a white man clutching his head as a black man carries him over his shoulders, flanked by police in riot gear, near Waterloo station.
Onlookers identified the injured man as far-right protester and said he was rescued by a Black Lives Matter activist as animosity was briefly set aside on a day of clashes in the capital.
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The man had been set upon on the steps leading to the Royal Festival Hall in central London and badly beaten, before other protesters stepped in to protect him, according to Reuters journalists at the scene.
Earlier in the day skirmishes broke out between anti-racism demonstrators and far-right protesters who took to the streets of London in response to recent Black Lives Matter rallies sparked by the police killing of George Floyd in Minneapolis last month.
Far-right groups shouted racial slurs at the anti-racism protesters, and some tried to use metal crash barriers to break through police lines.
The Metropolitan Police said it had arrested five people for offences including violent disorder and assault on police and that six officers had suffered minor injuries. Fifteen people were treated by paramedics.