Anti-terrorist branch officers are investigating a series of IRA bomb attacks in London before Christmas.
In Northern Ireland, Army bomb disposal experts were last night examining a car stopped by police in Belfast.
Police believed the car contained at least two bombs which were to be detonated in Belfast city centre.
The car was stopped at a random police checkpoint on the Newtownards Road in the south of the city. Examination of the vehicle revealed two suspicious devices. A Royal Ulster Constabulary spokesman said two men had been arrested.
In a separate terrorist attack, several policemen were injured when at least one mortar bomb was fired at an RUC station in the village of Clogher in Co Tyrone last night.
A spokesman for the Royal Ulster Constabulary said none of the casualties was believed to be seriously injured. The IRA claimed responsibility.
The area was sealed off and surrounding homes evacuated while a follow-up operation was mounted to search for any unexploded devices.