Members of Quebec's dominant construction union, FTQ-Construction, blocked access to some government buildings Tuesday morning, notably the Montreal headquarters of the Quebec Construction Commission.
About 30 members of the union plastered stickers reading "not fair" on the building and stopped workers from entering the parking lot for several hours Tuesday morning.
Police were called in to ask the union members to step aside, and access was gradually allowed to the building.
The union is upset about legislation tabled last week by the Charest government that strips unions of the right to decide which workers get access to job sites.
Bill 33, the government's attempt to curb the influence of organized crime in the province's construction sector, would also force unions to open their books to outside audits and public scrutiny.
"It's a diversion from other well-known problems," FTQ-Construction director general, Yves Ouellet, said, adding that the union has the right to express its views of the bill.
The union has also launched an ad campaign against the bill, saying it takes away its ability to represent members.
Speaking from Spain on Friday after unions blasted the proposed law, Quebec premier Jean Charest said his government would not back down and that the time had come to change the situation.
Previous initiatives to curb the influence of construction unions on job sites resulted in intimidation, according to former Labour Minister, Laurent Lessard.
Lessard, who now handles the Municipal Affairs portfolio, admitted he had been intimidated during parliamentary hearings in 2005 on union tactics at construction sites.
Quebec provincial police had to ramp up security at the time.