Objects were thrown at Nigel Farage as he rode an open-top bus in Yorkshire on Tuesday, 11 June.
The Reform UK leader was campaigning in Barnsley when a protester approached the vehicle.
Mr Farage said he believes objects were some wet cement from a work site followed by a coffee cup, and was warned by police not to get off the bus.
He had just addressed supporters but was interrupted by a crowd of demonstrators.
Police arrested a man who threw the objects.