Video footage shows a crowd of demonstrators carrying placards and chanting “Nazi scum” at the man in Parliament Square.
An object can then be seen flying through the air before hitting the man, sending what appears to be milkshake dripping down his forehead.
As the man prepares to throw a paper cup back at the crowd, a police officer standing beside him can be seen grabbing the radio on her vest. No other police can be seen nearby in the footage.
Matthew Thompson, an LBC journalist, shared the video on Twitter saying: “A Trump supporter is milkshaked by a hostile crowd in Parliament Square. And then it all kicks off...”
Labour leader Jeremy Corbyn accused the US president of spreading “hate” as he spoke at an anti-Trump rally in Westminster on Tuesday.
Mr Trump, who is in the UK with his family for a three-day state visit, spent Monday visiting the Royal Family at Buckingham palace and held a press conference with prime minister Theresa May on Tuesday.
Other “milkshaking” victims include Tommy Robinson, Nigel Farage and Carl Benjamin.
The trend appears to have taken off in the UK after Mr Robinson, a far-right campaigner, had milkshake thrown over him while campaigning in Warrington.
The anti-Muslim activist was repeatedly accosted by protesters armed with milkshakes during the campaign, as he tried to win votes while standing to be an MEP in the northwest of England.
Brexit Party leader Mr Farage and Carl Benjamin, a far-right YouTube personality, also had milkshakes thrown at them before the European elections.
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