The officer was shot dead just days after the murder of two soldiers in the British province.
The victim of Northern Ireland's third killing in as many days was a police officer responding to a call for assistance.
The British province's police chief says officers were fired on from waste ground nearby.
SOUNDBITE: Hugh Orde, Chief Constable of the Police Service of Northern Ireland, saying (English)
"The officers were fully aware of the threat they were facing. They did not stand back, they did not step back, they knew the person needed help. Mindful of the threat they responded in a totally appropriate way. Sadly one officer paid with his life protecting the service, protecting the community."
The killing on Monday came just two days after two British soldiers were shot dead at the entrance to their barracks in the province.
A small dissident splinter group calling itself the Real IRA which opposes to British rule has claimed responsibility.
It says it will reveal details of what it called its new operation in the coming days, raising fears of a resurgence of sectarian violence.
The group is shunned by the province's politicians who have put aside years of enmity to work together in a devolved national assembly.
Those who support British rule in the province and those who oppose it have united in appealing for calm.
They say the attacks will not derail the peace process launched almost 11 years ago after three decades of bloodshed.
Paul Chapman, Reuters
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