The device exploded under the vehicle outside his home in Highfield Close, off the Gortin Road in Omagh, Co Tyrone, just before 4pm today.
It is understood the 25-year-old was a new recruit to the Police Service of Northern Ireland and was a Catholic.
Shadow Northern Ireland secretary Shaun Woodward said: "This evil and cowardly attack will sicken everyone across Northern Ireland.
"These crimes are targeted on those who protect the community.
"We all deeply mourn the brave young man whose life was taken by this savage crime.
"We all have a duty to stop those behind it from succeeding."
Politicians north and south of the border condemned the bomb attack on the residential housing estate.
Sinn Fein president Gerry Adams said his party was determined that those responsible would not set back the progress of the peace and political process.
Tanaiste Eamon Gilmore, the Republic of Ireland's Foreign Affairs Minister and deputy leader, warned that those behind such violence have no mandate and are acting contrary to the democratic will of the people of Ireland, north and south.
The blast will send shivers through the people of Omagh, where 29 people, including a woman pregnant with twins, were killed when a Real IRA car bomb exploded in 1998.
Irish Taoiseach Enda Kenny described the bomb attack as a heinous and pointless act of terror.
© UnknownThe scene in the Highfield Close area of Omagh after a newly recruited PSNI officer was killed in an under-car bomb attack. April 2011
"Those who carried it out want to drag us back to the misery and pain of the past," he said.
"They are acting in defiance of the Irish people.
"They must know that they can never succeed in defeating the democratic will of the people."
It is believed the young officer was from Omagh and only graduated from police training college three weeks ago.
He was in the car alone when neighbours rushed to the scene seconds after the bomb exploded.
About 2,000 people taking part in the Omagh Half Marathon passed the nearby entrance to the estate just hours before the blast.
Ulster Unionist leader Tom Elliott said those behind the murder had one aim - to take Northern Ireland back to the dark days of the past.
"The deliberate targeting of a new recruit to the police by these criminals is utterly reprehensible," he said.
SDLP leader Margaret Ritchie said the policeman's killers were enemies of Ireland.
"This has not only stunned the people of Omagh, it has stunned the entire country," she said.
"This is not what the people want. They cannot be allowed to continue their campaign."
Meanwhile, party chairman Joe Byrne, who was at the scene within minutes, said the attack had stunned a community which had finally begun to come to terms with its past.
"Those responsible have no support in the town of Omagh. Nobody wants them," he warned.
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