A fire department hazardous materials team secured the package and determined that its contents were nonhazardous.
One police officer, three firefighters and three civilians were held in isolation briefly, although they showed no signs of ill health.
Another 40 to 50 people on the 15th floor of the building at 269 Laurier Ave. were asked to stay where they were while the matter was being investigated.
The rest of the office building was unaffected.
As of the lunch hour, police said they were continuing to investigate the incident.
The building sits just five blocks from Parliament Hill.
Police cars, fire trucks and paramedic vehicles blocked the street alongside the building, snarling traffic in the downtown core.