A second explosive device has been found amid rising sectarian tensions in advance of Hillary Clinton's visit to Northern Ireland on Friday.
Just hours after police intercepted a device which they believe was about to be planted by dissident republicans in Londonderry, police revealed the discovery of a letter bomb at a postbox in a loyalist area of Co Down.
Mrs Clinton is due to meet Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness at Stormont Castle, Belfast, before returning to the United States later on Friday afternoon.
Detectives are questioning four men about the discovery of the bomb in Derry's Creggan estate last night. They were arrested when a car was stopped.
Security chiefs believe dissident republicans opposed to the peace process were planning an attack in the centre of the city, next year's UK City of Culture.
The discovery of the letter bomb at Clough, a loyalist area near Newcastle, Co Down, was disclosed by police two hours before Mrs Clinton's arrival. It was also found last night when officers were alerted to suspicious activity near the postbox.
Clinton's husband Bill was a crucial part of the Northern Ireland peace process in 1998.
She has been to Northern Ireland several times before and will have been well advised before her arrival in Belfast today about increasing loyalist anger on the streets because of the decision by the city council to limit the flying of the Union flag at City Hall.
The discovery of another dissident republican bomb in Derry will also have caused alarm.
Pat Ramsey, an SDLP member of the Northern Ireland Assembly who lives in Derry, said people were angry at the disruption caused by this latest bomb alert.
He said: "Once again, the people of the Creggan are the victims of disruption, distress and anger. People are genuinely fed-up with this.
"People are first of all looking forward to Christmas with their families and then, next year, to the City of Culture. And this may have something to do with it because the dissidents have consistently said we will not have anything resembling normality during the UK City of Culture year.
"It was freezing cold last night and there were people in their 80s and 90s who were moved from their home. Some
"The people they tell us they represent are the people who are being disrupted. This has to stop.
"Thank God the police found the bomb and the people of this city or somewhere else are not waking up to destruction today."
Dissident republicans who were behind the murder of prison officer David Black last month have been particularly active in the Londonderry area in recent years.
They are opposed to the UK City of Culture status that has been bestowed on the city for next year. Attempts have been made to blow up the City of Culture offices on a number of occasions because it is seen as a highly symbolic target.
In March, dissidents tried to bomb Derry Courthouse and there have been attacks on police stations in the city. They have also been responsible for a number of murders and have forced young men to leave Derry.
Tensions in Northern Ireland have been high in recent days after a controversial vote on flying the Union flag over Belfast City Hall.
Loyalists have been holding protests across the country to show their opposition to the decision to restrict the number of days the flag is flown.
Councillors from the non-sectarian Alliance Party have been intimidated and in one case the party's offices were destroyed by fire.
Meanwhile, east Belfast Alliance MP Naomi Long pledged to resist loyalist attempts to force her to quit working in the constituency after police warned her early today there was a threat on her life.
She revealed they had advised her to leave her home and not return to her office on the Newtownards Road where loyalists staged a demonstration earlier this week in protest at the City Hall Union flag vote.
Ms Long, a former city councillor who won the seat at the last general election from Peter Robinson, said: "This will not prevent me providing a constituency service to the people who elected me."
Mr Robinson said the threat was an affront to democracy and regardless of political difference, public representatives should not be attacked or threatened in any way.
He said: "It is an attack on us all. As someone who in the past has been visited many times by the police to be told of death threats issued against me, I know how difficult and testing a time this is."