Montreal police have opened an investigation after demonstrators staged a mock beheading of an effigy of Quebec’s labour minister during a May Day rally, drawing widespread condemnation from political leaders. Video from the event shows protesters using a guillotine to cut the head off a papier-mâché figure of Jean Boulet before cheering and kicking it.The Montreal Police Service confirmed an investigation is underway, adding a “deep analysis” of the incident is being conducted.Chantal Rouleau, Minister Responsible for Montreal, called the act unacceptable. “It’s terrible that people can do that,” she said, adding “those people must excuse themselves. We are not like…