The break occurred in the 8400 block of Connecticut, near the northbound lanes of the road, according to WSSC spokesman Jerry Irvine. The cause of the break had not been determined, he said.
Connecticut Avenue was closed Monday night for about a mile on either side of the break.
As of late Tuesday, no injuries or structural damage had been reported. There was no report of any evacuation from two apartment houses near the scene, Irvine said.
However, the extent of any effect on water service as a result of the continuing drainage was not immediately clear.
There had been no indication that water service had been cut off in any part of the WSSC service area, Irvine said. However, he said, low water pressure had been reported.
Callers to the utility were cautioned Monday night that discolored water and reduced pressure might be observed as far from the break as Silver Spring.
One factor that appeared to reduce the potential for disruption was the direction of flow of the water from the main.
Irvine said it was spilling into a nearby creek bed, not into buildings in the area. A witness said it appeared that flow at a rapid rate was occurring into a ditch between buildings.
The roar of the water escaping from the main could be heard late into the night.
Although the valves that control flow through the main had been located, operating them would take some time, Irvine said. The size of the main precluded rapid shutdown of the valves.
No firm estimates were available about when the main could be off. It was also unclear to what extent traffic on Connecticut, a major commuter artery, would be permitted Tuesday morning.