There is a fire at a smaller building on the grounds of the Natural History building. Probably a building that houses electrical equipment.
This incident has Constitution Avenue closed going eastbound starting at 9th Street.
The 9th Street tunnel is also closed and 7th street southbound is only open with one lane.
UPDATE: The fire is now under control. None of the exhibits were damaged. The streets are opening back up around the museum. Constitution and 9th are both open now. Madison remains closed for about another half-hour (will be open about 11 a.m.).
A great picture from twitter user @kimberlyfaye is available here: http://www.flickr.com/photos/kimberlyfaye/5424416723/sizes/l/in/photostream/
Another great picture from DCist: http://dcist.com/2011/02/fire_inside_national_history_museum.php