Three men have been arrested after a bomb was found under the car of a serving police officer.
Police remain at the scene of the security alert in Glenrandel in Eglinton near Derry.
Police were alerted to suspicious activity outside a house at around 2.45am on Thursday morning and when they arrived they found the device under a car parked out a house.
Fifteen houses have been evacuated during the alert.
The suspects, aged in their late 20s and late 30s, were detained just across the border in Ballybofey, Co Donegal, at about 4am this morning.
A Garda spokeswoman said the men were being held at stations in Letterkenny and Milford.
The car will also be subject to a technical examination and searches are continuing in the vicinity of where it was stopped, she added.
Army bomb disposal experts were sent to the scene and Superintendent Mark McEwan confirmed it was an "Under Vehicle Improvised Explosive Device capable of causing death or serious injury" under the car of a serving police officer.
He said: “This device had the potential to harm not only the officer but anyone in that residential area and those involved have shown a blatant disregard for human life. This is yet another example of the cowardly elements in our society who show no regard for the safety of local residents and the police officers who serve their community.
“We are obviously very thankful that this attack was thwarted and that no one was injured here this morning but this is a stark reminder that the threat against our Police Service and its employees remains at severe.
“Clearly there are people out there in today’s society who are still intent on causing murder and mayhem. These criminal elements are not welcome in our society and I am appealing to the local community for their assistance in driving this small minority from our streets and bringing them before the courts.
“Glenrandel remains closed to traffic and residents and approximately 15 houses had to be evacuated during the early hours. I would like to take this opportunity to thank the local residents for their patience and co-operation and assure them that we are taking every sensible step to keep them safe and minimise any further impact on them.
“The investigation is at an early stage and we would appeal for anyone who may have been in this area around 2.45am this morning or in recent days who may have witnessed anything untoward or noticed any persons acting in a suspicious manner.
“We are also appealing for anyone who may have witnessed any vehicles seen leaving the area at speed shortly after 2.45am to contact us. Anyone with information should contact detectives at Strand Road on 101 or alternatively contact Crimestoppers on 0800 555 111.”
First Minister Peter Robinson said it was "vital" those responsible were brought to justice and urged anyone with information to pass it to police.
Mr Robinson said: "This was a clear attempt to murder a police officer who is working for the good of everyone in our community. It was only through vigilance that this device was discovered and a potential tragedy averted.
"Everyone must stand united against those who would use violence and terror in our society and I unreservedly condemn those behind this incident.
"It is vital that those responsible are identified and brought to justice and I would urge anyone with information to pass it to the police."
Mark Lindsay, chairman of the Police Federation for Northern Ireland (PFNI), described the planting of the bomb as an act of madness.
He said: "There is a network of people responsible for this act of madness. They obtained the components for the device. They made the device, targeted the officer and planted it under the officer's car in a cold-blooded attempt to murder.
"They didn't care who else they killed or injured. Their only objective is to drag us back to darkness and hopelessness and I want to appeal to the community to help the police track down the culprits of this cowardly attack."
The region's Justice Minister David Ford added his condemnation of the attack.
He said: "Clearly, whoever attached this device to the police officer's car had intended it to murder a servant of the community. I want to praise the vigilance of the owners who have averted a tragedy.
"I unreservedly condemn the people who hide in darkness and seek to use extreme violence. No rational person can see that as a way forward."
SDLP Foyle MP Mark Durkan has strongly condemned the "evil agenda" of those responsible.
He said: “I utterly condemn this attack – carried out by people steeped in the mind-set and means of past violence.
“These people are capable of using a viable device, but they don't actually have a viable political strategy, no coherent message and it is not what the community want.
“Let's be very clear about the support and sympathy and solidarity that we are offering to the police officer concerned.
“Indeed, it is vital we stand strong and united today in offering our support to the entire policing family after such an attack – which demonstrates just how vulnerable any individual can be.
“The real patriots are police officers serving all in the community, not those intent on plunging us back to the ugliness and helplessness of the past.
“No matter what other issues, differences or difficulties there are, one solid and compelling message that must go out to so-called dissidents is that we are not standing still. We are moving forward on the path of democracy and reconciliation that we have decided upon, and we will take the necessary decisions and steps to bring that to a conclusion and to defeat their evil and negative agenda.”
Foyle DUP MLA Gary Middleton has condemned those responsible and called it "disgraceful".
He said: "This is a disgraceful attempt to murder a police officer who serves our community. Such acts of terrorism must be condemned because they are wrong, and not simply because those carrying them out have no support for their actions.
"People in the Eglinton area are shocked by this attempt to bring death and destruction into their community and I know that everyone will stand in solidarity with this officer and his family.
"It is vital that those responsible are identified and brought to justice.’
Chair of the Assembly’s Justice Committee Alastair Ross added: "This is a sad reminder that there are still those in the community who's exclusive aim is to drag us back to the past, target those who seek to protect us and continue a futile terror campaign that delivers nothing for anyone in Northern Ireland.
"It is important that anyone with information on this attempted murder gives it to the PSNI in order to protect the public and put those responsible behind bars."
Alliance Policing Board member Trevor Lunn said he was "disgusted".
The MLA said: “I am disgusted by those who have tried to take the life of a Police Officer. These terrorists have no place in our society and their actions must be condemned by everyone. They want to drag us back to the dark days of the Troubles.
“I would like to send my support to this officer. It must have been a terrifying ordeal to discover that they had been targeted in this way.
“The vast majority of people want to move forward and help build a shared future for everyone in our society.
“The people behind this bomb are very dangerous and must be urgently apprehended by the Police, so I would urge anybody with any information to contact the PSNI or Crimestoppers.”
Sinn Fein MLA Raymond McCartney said death or serious injury was narrowly averted.
Mr McCartney said: "Thankfully no one was injured in this attack but we could have been facing serious injuries or worse.
"Local residents, many of them older people, had to be moved out of their homes as a result of this incident and had to spend the night elsewhere.
"This is a city moving forward and the vast majority of people do not want these type of attacks and do not support them.
"Those responsible are not representative of the people of Derry and won't be allowed to drag us back to the past."
In a joint statement, Derry's four main church leaders said the attack was wrong, evil and reckless.
Bishop Donal McKeown, Bishop Ken Good, Dr Robert Buick and the Rev Peter Murray said: "We are appalled at the despicable attempt to murder a police officer in Eglinton this morning, and thankful and relieved that the attack was thwarted.
"There can be no justification for the attempted murder. It was wrong. It was evil. It was reckless. The police have said those who left the device endangered not only the police officer but other people living in the area."
Northern Ireland Secretary Theresa Villiers added condemnation and urged anyone with information to contact the police.
She said: "I utterly condemn this cowardly terrorist attack on police officers who serve the whole community.
"Those responsible act in defiance of the wishes of the vast majority of people who want to see a peaceful and prosperous future for Northern Ireland."