A woman died and dozens of people were injured after thick smoke filled a subway tunnel in the US capital during Monday's evening rush hour, officials said.

A spokeswoman for the Washington Metropolitan Area Transit Authority confirmed the incident that caused mayhem in the second-busiest US mass transit system after New York.

Washington Emergency Medical Services said 84 people were taken to the hospital with "various injuries" and one firefighter was also injured. Some 200 people were being evaluated.