The incident took place when the constable, Vikram, who is posted at Mayur Vihar police station, and a colleague had gone to inquire about an incident in the Kalyanpuri area. Neither policeman was in uniform.
On their way back, Vikram's motorbike brushed another two- wheeler following which the other rider allegedly stabbed him with a knife, a senior police official said.
The accused, identified as one Sunil, turned out to be a proclaimed offender and was arrested after the incident.
Vikram, who received an injury on his thigh, was taken to GTB Hospital for treatment, the official added.