Grove Road is closed both ways near Portland Prison while a crane is being operated between Shepherds Croft and Incline Road.
The diversion is not thought suitable for larger vehicles.
The road is due to re-open at 6pm.
More top news
Only handful of beaches have RNLI lifeguard cover during school holidays - the Maritime and Coastguard Agency warns.
Our Devon correspondent Seth Conway has been talking to a family who say without the hospitals, their children may not have survived.
It's going to be a windy one tonight and into the weekend - it could even touch gale force along the south coast.