Union activist Nicole Arow, 28, said she was "thrilled and relieved" to learn about the incumbent's win. "Joy. That's what I feel."
Those who arrived at the White House shortly after US television networks called the race for Obama appeared to be mainly college students, but the party quickly grew to include middle-aged people and parents with young children.
Downtown Washington, usually deserted at midnight on a weekday, was teeming with cars, drivers honking their horns in celebration and waving US flags.
Justin Pinn, a 22-year-old student at Georgetown University, gushed : "My hope is renewed. I'm ready to fight the good fight. It's a great day to be an American!"