Traffic is building on the M275 southbound and on the main roads leading towards Southsea Common as runners try to make their way to the official car parks.
Road closures are currently in place across Southsea, with more in place from 9.45am ahead of the start of the Great South Run.
The first elite athletes will start at 10.15am with the mass start from 10.30am.
Drivers have been advised to use alternative routes, with both Eastern Road and Eastney Road currently free-flowing.
Traffic is particularly slow on the A3 Marketway as drivers try to park in the official Southsea Common car park.
Stagecoach South has said that the 23 service will be terminating at the Hard interchange from 7.55am, and then at the city centre from 9.45am.
Hovertravel is putting on additional services between Ryde and Southsea – including at 9am and 9.30am.
The car park at Gunwharf Quays is also closed from 9.30am to 12.30pm, with visitors encouraged to use the park and ride service.