The protesters, members of the Industrial Workers of the World union, entered the non-commercial building at 7 a.m., but were promptly escorted out. The union does not represent mail carriers, but organized the protest to show solidarity with the workers.
They continued the protest outside the Repentigny sorting centre.
Demonstrators blocked mail trucks from leaving the facility's parking lot until just before 10 a.m. when police forced them off the property.
The protest comes a day after a federal court challenge was launched to try to reverse Canada Post's decision to end door-to-door delivery.