Irish Republican Army dissidents left a 400-pound car bomb outside police headquarters in Belfast but the homemade device failed to detonate, Northern Ireland's police commander said.
As politicians warned of a rising threat from IRA diehards, three other suspected IRA dissidents were arrested Sunday in another attack on police.
Chief Constable Matt Baggott said Saturday night's abortive bomb attack on the Northern Ireland Policing Board office in Belfast's docklands represented an attack on the province's entire peace process.
That process has created a joint Catholic-Protestant government and growing support for law and order, achievements that the dissidents hope to undermine.
The explosives-laden car caught on fire but didn't explode and caused no damage to the Policing Board building, where a cross-community panel oversees police operations.
Security guards said they saw two men running away. The attackers' suspected getaway vehicle was later found burning - to destroy forensic evidence - in the nearby New Lodge district, a poor housing project long known as an IRA stronghold.
Around the same time Saturday, a police patrol came under gun attack in Garrison, a lakeside border village in the westernmost corner of Northern Ireland. Baggott said police returned fire with two shots, but nobody was hit.
Two suspected IRA dissidents were arrested near Garrison and a third across the border in the Republic of Ireland on suspicion of involvement in the shooting.
A moderate Catholic member of the Policing Board, Alex Attwood, said the IRA dissidents "are broadening the scale of their attacks on democracy."