The break is near the corner of Barclay and Westbury avenues.
Firefighters got the call about the break at 4:11 a.m. when the 20 cm-pipe started gushing water onto the street.
About 40 firefighters are on the scene, setting up booms to divert the water and helping with evacuations.
Four families had to be evacuated from two apartment buildings. About 10 basements were flooded.