Wellington Street is closed to all traffic in both directions from O'Connor to Elgin streets.
The call at 80 Wellingston St. came in after 11 a.m. ET.
A suspicious male was reported in the area, and he was apprehended outside, RCMP said.
The Langevin building is across the street from Parliament Hill and houses the Privy Council offices and Prime Minister Stephen Harper's office.
Harper, who also has an office in Centre Block on Parliament Hill, was not in the Langevin building at the time of the threat.
The CBC's Tom Parry reports a police explosives unit truck has been stationed in front of the Metcalfe Street entrance on the west side of the building.
Police have moved media further back across the street.
More to come