In June, a video showing a forceful arrest of a Canadian indigenous leader by the Royal Canadian Mounted Police raised questions about the use of force by the police.
The statue of Macdonald has been the site of repeated acts of graffiti in recent years, and it has often been covered in red paint.
Saturday's incident drew quick condemnation from political leaders.
"Whatever one might think of John A. Macdonald, destroying a monument in this way is unacceptable," Quebec Premier François Legault said in a tweet. "We must fight racism, but destroying parts of our history is not the solution."
Newly elected leader of Canada's main opposition Conservative Party, Erin O'Toole, said, "We will not build a better future by defacing our past."
- Reporting by Denny Thomas in Toronto