"Detectives investigating last night's explosion in Derry/Londonderry have arrested two men. The pair were detained in the city this morning and are being questioned under Terrorism Act," the statement read.
PSNI Assistant Chief Constable Mark Hamilton said Irish Republican Army dissidents were suspected of being behind the attack.
On Saturday, a car bomb exploded in Londonderry at around 8:10 p.m. local time (20:10 GMT). Police said they were checking a suspicious vehicle when they received information that a device had been left at the courthouse. It was found that the car used had been hijacked from a delivery driver. No one was injured during the attack.