The statue, erected in 1735, is under 24-hour watch amid fears it will become a flashpoint for trouble.

Recent research has found that “King Billy” benefited personally from the slave trade. The Orange Order have warned that any attempt to move the statue would be “totally wrong”.

Jim McHarg, grand master of the Grand Orange Lodge of Scotland, said the vandalism was upsetting and called for the police to find the culprits.

“King William brought liberty and took away the divine right of kings and the tyranny associated with that. It is very strange to hear he is connected with slavery. He is part of the history of Great Britain.

“It has nothing to do with the people who were protesting for the right reasons. There are people with an agenda to attack the protestant people of Scotland and Glasgow. This has come from republican and socialist people who hate everything that is unionist,” he said.

Kenny MacDonald, assistant chief constable at Police Scotland said: “I am aware that vandalism of a small number of monuments and statues has taken place in recent days. Such action is criminal and will be fully investigated. Police officers will be providing additional patrolling around such monuments and statues to deter such criminality.

“We understand that people want to make their voices heard, but they must do so lawfully and peacefully.”

He added: “There will be an appropriate policing plan in place for the coming days and into the weekend.”

Equality campaigners have also pointed out that the statue of the Duke of Wellington, which stands outside the city’s slavery-funded Museum of Modern Art on Queen Street, features friezes of the duke’s colonialist exploits. A longstanding Glasgow tradition sees a traffic cone placed on his head regularly, which is believed to cost the council £10,000 a year to remove.

An attempt by the council to double the height of the statue’s plinth in 2013 was met with fierce public opposition and a Facebook campaign called “Keep the Cone”.

A pro-cone rally was thereafter organised to defend the practice. It included signs with slogans such as “Coney No Dae That”, and “We Are All The Glasgow Cone”.