The break sent a geyser of water into the air and quickly flooded nearby streets.
Lower pressure was reported in the south end of the city, specifically in some high rise buildings. Firefighters were concerned about low water pressure in the area.
People in Jersey City have been told to boil their water for one minute before using it to do anything.
The break caused early morning traffic delays on Route 7 during repairs.
No homes were reported without water.
United Water is working to fix the problem.
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