The letter bomb sent to the PSNI chief constable George Hamilton at police headquarters was “reckless and indiscriminate” the MP for the constituency has said.
The device was defused at the east Belfast headquarters of the PSNI by bomb disposal experts.
The PSNI could not confirm or deny reports that the package was posted in the Republic of Ireland.
The alert was raised after a suspicious package in a brown padded bag with a white address label was received through the post at the station in Knock on Thursday.
Alliance East Belfast MP Naomi Long said: “There can never be any justification for such acts of terrorism.
“It was clearly the intent of those who sent the device to seriously injure or kill someone. Sending a letter bomb is a reckless and indiscriminate act and those behind it are beneath contempt.
“I would like to praise the brave actions of the police and Army officers who dealt with this device. Their bravery stands in stark contrast to those evil individuals behind this bomb.
“The vast majority of people in Northern Ireland are opposed to the actions of those who sent this device. Those responsible must be caught by the police, so I would urge anybody with any information to contact the PSNI or Crimestoppers.”
Superintendent Sam Donaldson also said that someone could have been seriously hurt.
“This is a mindless and foolish act that could have resulted in the serious injury of any person handling the package,” he said.
There is no information about any other packages, he said, urging people to be vigilant.