A total of 45 firefighters put out a blaze at a set of flats by Gunwharf Quays this afternoon.
Ten fire appliances rushed to the fire in a four-storey flat above the Tesco Express store next to the shopping centre.
The blaze is believed to have started in the kitchen.
There were no serious injuries to any flat occupants though three people are believed to have been treated for smoke inhalation.
The service was called to the scene at 12.58pm with the building subsequently evacuated.
Crowds gathered outside the building while firefighters fought the flames and the blaze was put out at around 2.30pm today.
Deputy Chief Neil Odin of Hampshire Fire and Rescue Service said: ‘The flat is severely damaged. It has been absolutely gutted but luckily it was the only flat that was internally affected by the fire.
‘There are no serious injuries as a result of the incident but two people were subsequently treated for light smoke inhalation.
‘An investigation has now been launched into the cause of the fire which is not yet known at this stage.’
Appliances were in attendance at the incident from Cosham, Southsea, Havant, Fareham and Eastleigh.
Eyewitness Lyn Murphy, who was walking past the flat when smoke started coming out of the flat, said: ‘It was just a bit of smoke at first but then there was a lot of it.
‘I could see flames in the flat.’
A South Central Ambulance Service spokeswoman said they were called at 1.20pm and treated a patient, although they did not require hospital treatment.
Police are on the scene and all roads in the area that were closed have now been re-opened.
The shopping centre remains open to visitors, but the car park is closed.