Montgomery County fire officials said five people were checked for hypothermia. The temperatures outside were in the 20s.
"The water tumbled over the car like a wave," said Silvia Saldana, of Springfield, Va., who was traveling to work when she became trapped. "I started to pray."
She also called her husband three times while she was stuck in the car for about 30 minutes before help arrived.
Fire spokesman Pete Piringer said crews had trouble getting to people because of the swift-moving water. Officials said 150,000 gallons of water per minute were gushing out at one point, spilling debris onto the road.
Piringer said some trees fell onto a power line and knocked down a utility poll. Schools in the county closed early because of widespread water outages, and a hospital where three people rescued were treated and released diverted ambulances and closed its trauma division because of lost water pressure.
A man who lives about 50 feet from the street described the immediate, unexpected flood after the pipe, about 5½ feet in diameter, ruptured.
"I thought it might be a minor leak, then suddenly I stepped outside and, 'My God!"' said Raj Bhansaly. "It looked literally like the Potomac River."
From his house, Bhansaly said he saw two cars tied to rescue vehicles with ropes.
The water main broke about 8 a.m. on River Road near the Congressional Country Club in Bethesda, authorities said.
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. Authorities said the water has flowed into a nearby creek.
There have been several major water main breaks this year in the wealthy suburb of Montgomery County. In June, a rupture closed more than 800 restaurants and left tens of thousands of people scrambling for clean drinking water.
The Washington Suburban Sanitation Commission has warned its system is aging, overtaxed and underfunded. It serves 1.8 million suburban Maryland customers and has had an increasing number of water main breaks, including 1,357 between January and November this year. Last year, it had a record 2,129 breaks or leaks.
White said the pipe that broke Tuesday was installed in 1964.
"We're plagued by old pipes," White said. "Throughout the nation, aging infrastructure is a problem."
Associated Press writers Brian Westley and Nafeesa Syeed in Washington and AP photographer Jacquelyn Martin in Bethesda contributed to this story.