The incident happened as a crowd of Stand Up to Racism protestors interrupted Mr Nigel Farage as he spoke from the vehicle.
Reform UK supporters and Stand Up to Racism campaigners both chanted against each other as crowds formed around the bus in the town centre.
Mr Farage had been warned by police not to get off the bus.
He had been touring Barnsley and had just left Hoyland, where he had had a warmer welcome. He called in at a beauticians and posed with staff while joking about having his nails painted.
Mr Farage also spoke with shoppers and while standing by a market stall selling eggs, joked that after being covered in a milkshake last week he did not want eggs to be the next thing thrown at him.
Last week, Mr Farage had a banana milkshake thrown at him while campaigning in Clacton.
A woman was later charged with assault.
During a campaign walkabout in Newcastle in 2019, he also had banana and salted caramel milkshake thrown at him.
