A device has exploded outside the offices of the nationalist SDLP in north Belfast.
Police said there were no immediate reports of injuries.
The attack happened close to the Antrim Road, near Duncairn Gardens. An army bomb disposal team has been alerted to go to the scene.
North Belfast Assembly member Alban Maginness, whose constituency offices suffered damage in the blast said four members of a scout group who rent the premises were in the building at the time.
Mr Maginness said it was "just by the grace of God" that no one was injured.
"The fact that there was a steel door that deflected so that the blast went out rather than in saved them.
"The four people who were upstairs would have been injured and possibly killed."
Mr Maginness added that it was "a callous, cowardly attack".
The bomb attack was condemned by the Ulster Unionist MP for North Belfast, Cecil Walker.
"I and my colleagues in North Belfast unreservedly condemn this attack which was deliberately designed to kill and seriously injure," he said.
"I am disgusted that once again the name of North Belfast is being blackened. "