A 28-year old man has been arrested on suspicion of urinating on the memorial of a murdered policeman during far-right protests in central London on Saturday, the Metropolitan Police said.
The man is in custody in Essex, east of London, after presenting himself at a police station, the police said.
Far-right protesters clashed in London on Saturday with anti-racist demonstrators and with police trying to keep the two sides apart.
Prime Minister Boris Johnson condemned the violence, saying "racist thuggery has no place on our streets".
The Westminster memorial honours PC Keith Palmer who was murdered while on duty at the Houses of Parliament during an attack in 2017 in which four other people were also killed.
Met Police Commander Bas Javid said on Saturday: "We are aware of a disgusting and abhorrent image circulating on social media of a man appearing to urinate on a memorial to PC Palmer.
"I feel for PC Palmer's family, friends and colleagues. We have immediately launched an investigation, and will gather all the evidence available to us and take appropriate action."