The officer was stabbed in the stairwell of a metro station at Medborgarplatsen, officials said.
The entire square, which is popular with tourists, and a nearby nursery have been sealed off by officers. A manhunt is underway.
Witnesses on social media said the attacker “snuck up behind the policeman” and stabbed him “unprovoked”.
The nursery is attended by 70 children. There are no indications the building is in danger.
Stockholm police said: “We have a policeman who has been hurt at Medborgarplatsen.
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“We have a number of patrols.”
A witness said: “An ambulance has been here … it is unclear if it was with the police officer who was hurt.”
Police were alerted to the stabbing at 10.30am local time (11.30am BST).
This is a breaking story, more to follow…