The policemen were returning after visiting an abattoir in suburban Govandi in connection with the investigation of a case when they saw Istiyak Sheikh (35) stabbing Faiyaz Qureshi (35) near the Govandi railway station, police said.
The incident took place at around 2.05 PM.
Sheikh was overpowered and taken into custody while Qureshi was rushed to a hospital by the police personnel, they said.
Sheikh was handed over to Deonar police station where a case of attempt to murder was registered against him.
Both Qureshi and Sheikh are residents of Gautam Nagar in Govandi, police said, adding Sheikh was a history-sheeter with several cases lodged against him at the Deonar police station.
The motive behind the attack was not yet known and the probe was on, they said.