The shooting took place on Crumlin Road in northern Belfast shortly after 7.30pm.
Witnesses reported hearing multiple gunshots from a passing vehicle before seeing the officer crumple to the floor.
It is understood the police officer was struck once in the arm.
Terrorists have gunned down a police officer outside a Northern Ireland petrol station
Police Service of Northern Ireland (PSNI) assistant chief constable Mark Hamilton said: "This was a very concerted attack on our officers – a very concerted attempt to murder them.
"Our main line of enquiry at the minute is violent dissident republicans.
"One arrest has already been made."
The Police Federation for Northern Ireland tweeted that "an officer had been injured in a terrorist shooting" – the terror level in the country was raised to "severe" by the British Government in April, 2016.
Emergency services swarmed to the scene on Crumlin Road
Mark Lindsay, chairman of the Police Federation for Northern Ireland, said the "outrageous" terrorist act was committed by people determined to bring Northern Ireland to its knees.
He said: "This is an appalling act, and I wish to condemn it in the strongest possible terms.
"This attack on the life of an officer is a stark reminder of the determination on the part of terrorists to murder and maim police officers.
"They believe that by causing great grief to a family they are somehow advancing their warped and outdated plan. The wider community will be outraged by this attack on one of their police officers.
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"The police serve the entire community and wounding one individual is an attack on the entire community."
Mr Lindsay urged officers to remain vigilant.
He added: "It's one reason why the threat level here remains classed 'severe' which means a terrorist attack is highly likely.
"Officers already exercise great vigilance and I would appeal to them to remain at high alert."
Northern Irish police Tweeted that it was an act of terror
Northern Ireland justice minister Claire Sugden called the attack as "sickening".
Police have closed the Crumlin Road because they are still investigating the shooting.
Arlene Foster, leader of the DUP, called the attempted killing "the work of cowards".
Senior Sinn Fein assembly member Gerry Kelly said: "I hope that the injured officer will make a full and speedy recovery.
"Those responsible for this attack are the same people who are attacking the local community.
"They have absolutely nothing to offer society and need to call a halt to these activities immediately.
"I would call on anyone with information on this incident to bring it forward to the PSNI."
This is the petrol station where a police officer was shot by terrorists
James Brokenshire, Secretary of State for Northern Ireland, made these comments
The attack comes as Northern Ireland prepares for fresh elections after the collapse of powersharing.
Former deputy first minister Martin McGuinness resigned in protest over a botched green energy scheme which is predicted to leave taxpayers millions out of pocket.
Dissident republicans opposed to peace have killed police, prison officers and soldiers in the past.