The blast occurred minutes after the devolved power-sharing administration in Belfast resumed control over policing and justice in the province after almost 40 years, in a major step in the peace process here.
"A device has exploded in a vehicle at the rear of Palace Barracks in Holywood," a police spokeswoman said.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The barracks is a former British army complex just outside Belfast which now houses MI5.
The explosion occurred shortly after midnight, at about 2324 GMT Sunday, she said, adding: "There are no reports of any serious injuries at this stage."
Local journalist Brian Rowan told AFP the explosion "shook my front door".
"It appears that the seat of the explosion was on the opposite side of the complex in a layby on Old Holywood Road. The security services have sealed off a stretch of the road covering several miles," he said.
Hundreds of people are employed in the MI5 complex monitoring domestic security across the British-ruled province.
Northern Ireland endured three decades of civil strife between Catholics who wanted the province to become part of the Republic of Ireland and Protestants who wanted to stay within the United Kingdom.
The violence largely ended with the signing of the 1998 Good Friday peace accords, which paved the way for the current power-sharing administration between the Protestant DUP and the Catholic Sinn Fein parties.
Monday's transfer of police and justice powers was one of the last stages in the process of restoring normality to the province.
