Fire spokesman Pete Piringer said crews had trouble getting to people because of the swift-moving water.
Officials said that at one point, water was gush at a rate of 511 million litres per minute.
The water main broke during the morning commute on River Road near the Congressional Country Club in Bethesda on Tuesday, authorities said.
Montgomery County schools were closing about two and a half hours early because the main break caused widespread water outages across part of the county.
John White, a Washington Suburban Sanitary Commission spokesman, said it was not yet clear what caused the break.
Because of the gushing water's intensity, fire officials did not allow utility workers to immediately shut down valves where the break occurred, White said.
But crews were able to shut down two valves farther down the pipeline, slowing the flow.
The Washington Suburban Sanitation Commission has said that its system is ageing, overtaxed and underfunded.
It serves 1.8 million suburban Maryland customers and has had an increasing number of water pipe breaks, including 1,357 between January and November this year. Last year, it had a record 2,129 breaks or leaks.