Reagan airport was shut down for the night, most flights from Baltimore were cancelled and there were long delays for passengers using Philadelphia and New York.
Roads have been badly affected, too, with one transport official calling it a very serious storm.
As Washington's Union Station filled with travellers, some of them sprawled out on the floor, the passenger train service Amtrak said delays between Washington and Boston were averaging between 30 and 60 minutes.
At least two trains to Boston apparently departed more than four hours late.
Snow was being cleared from the M&T Bank Stadium in Baltimore ahead of an NFL game on Sunday
In Virginia, several hundred motorists became stranded in their vehicles and had to be rescued by the National Guard, using Humvees.
I'm going to do snowmen and sledding... and see the monuments in the snow
Some 500 people sought warmth and refuge in emergency shelters.
Three people died in the state. One was killed when a car hit a tree, a second died of exposure and a third was also apparently killed in a road traffic accident.
In Ohio, two people were killed in accidents on snow-covered roads hit by the same storm system.
The US National Weather Service warned that weather conditions in the Washington area had made travel "extremely treacherous".
"Do not travel," it warned drivers.
"If you must travel, have a winter survival kit with you. If you get stranded, stay with your vehicle."
Washington Mayor Adrian Fenty asked residents to sit out the weekend snowstorm at home where possible.
US President Barack Obama, arriving back at Andrews Air Force Base in Maryland from the Copenhagen climate conference, travelled into Washington by motorcade rather than helicopter because of the weather.
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