Motorists on Brighton seafront face a week of delays as a ring of steel is built around venues for the Labour Party conference.
Sussex Police have created a "sterile island" in the city centre, ringed with metal crash barriers.
The barriers jut into Kings Road, closing one of the two east-bound carriageways, leading to jams which have stretched more than a mile at peak times.
It is the first time such barriers have been used during the conference and they are part of a package of measures to make security at this year's event tougher than ever.
A motorist said: "It's bad enough when both lanes are open. Now it's an absolute nightmare."
The conference has been cut short in the wake of the terror attacks on the United States. It now runs from Sunday September 30 to Wednesday October 3.
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