The blackout is affecting dozens of roads just south of Fratton Park, in the PO4 postcode area, beginning at Goldsmith Avenue, Southsea, and extending south to Brompton Road.
The outage was first reported 5.30am and was one of several first raised across the city.
Engineers from Scottish and Southern Electricity Networks (SSEN) have been scrambled to try and restore power.
However, the energy firm has warned customers that power isn’t expected to be restored for hours.
In a statement on its website, the company added: ‘We’re sorry for the loss of supply. We currently have a fault affecting the areas listed.
‘If you need more information, please call us on 105, or send us a message on Facebook or Twitter (@SSENcommunity) and quote reference “JJ6748”.
SSEN hopes to have power fully restored to the area by 4.30pm.