Norfolk Constabulary said it happened on the A10 junction with Runcton Road at Stow Bardolph, near Downham Market, at around 7am on Tuesday.
The road is closed in both directions from Stow Bardolph to South Runcton.
The force said it is likely to remain this way “for some time” and urged motorists to find alternative routes.
According to the AA Traffic Map there is congestion in both directions and motorists are facing delays of about 20 minutes.
The extent of any injuries currently remains unknown.