A car bomb has exploded near an Army base in Holywood, County Down, forcing the evacuation of several homes, BBC reported.
It happened outside Palace Barracks in the town, where MI5 has its Northern Ireland headquarters.
Local people say they heard a large bang just after midnight. There are no reports so far of any injuries.
The blast seems to have been timed to coincide with the precise moment that policing and justice powers devolved from Westminster to Stormont.
Families were spending the night in a local community centre.
Police said the incident was ongoing, and sources said the bomb was taken to the barracks in a hijacked taxi after the cab driver's family was held hostage, the Press Association reported.
Justice transfer
In March, Northern Ireland Assembly members voted in favour of the transfer.
Out of the 105 votes cast in the Assembly, a total of 88 supported the move, with 17 Ulster Unionists voting against.
Disagreement on the timing of the devolution of the justice powers had threatened to collapse Northern Ireland's power-sharing administration.
Later on Monday, Stormont assembly members are due to vote on appointing a new justice minister - the first Northern Ireland politician to take responsibility for justice and policing in 38 years.