A Toowoomba doctor and anti-marriage equality campaigner has called for a more reasoned debate after his practice was vandalised.

President of the Australian Marriage Forum David van Gend said the word "Bigot" was painted in red spray paint on the side of his office sometime over the weekend, along with his surname in black paint.

The vandalised side of David van Gend's family medical practice in Toowoomba.

An encircled letter A – a common symbol for anarchists – was also left on the building, which Dr van Gend rents.

The vandalism has been reported to police.