'The police are in attendance at the scene.
During the alert emergency vehicles from the Scottish fire and rescue service attended the scene, and the area around the building was cordoned off.
The MSP building in Holyrood, Edinburgh, has been cleared following the incident.
A second envelope, which is not believed to have been opened, was sent to MSP Edward Mountain.
The "suspicious package" which led to Scottish Parliament being evacuated has been confirmed to be event invitations to Tory MSPs.
"It's a white powder incident, there were a couple of suspicious packages sent to a Tory (Conservative) MSP", a source at the parliament told Reuters, using the acronym for Member of the Scottish parliament.
'No other areas of the campus are now affected and we remain open to the public at present.More news: Santi Cazorla: I almost had to have my foot amputated
"The MSP block is now open and embers and their staff can return to their offices".
"Our incident management team in consultation with the emergency services are reviewing whether further measures are necessary".
In a statement, Mr Halcro Johnston said: "Part of the Scottish Parliament building was evacuated earlier today after a package containing a suspicious substance was opened in my office and was reported by my staff to the parliamentary authorities".
"This is part of a pre-arranged process put in place, ready to deal with such developing situations".
'The building has been evacuated as a precaution and inquiries are continuing'.
"The public are thanked for their patience and support during this matter".