More than 50 firefighters from across Hampshire were scrambled to the blaze on a top of the four-storey apartment block in Grafton Street at 4pm on Wednesday.
The huge fire led to a large number of flats being evacuated but no residents were injured or required treatment.
Yesterday police arrested a 51-year-old man from Portsmouth on suspicion of attempting arson with intent to endanger life, and officers have now made a second arrest.
A 30-year-old woman from Portsmouth has been arrested over the same offence.
Both she and the man have been released from police custody on conditional bail while police enquiries continue, a spokeswoman from Hampshire Constabulary said.
The spokeswoman added: ‘An investigation into this incident is ongoing and we would still like to hear from anyone who can assist our enquiries.
‘We would like to hear from anyone who may have any information about the fire, or who saw or heard anything suspicious on Wednesday afternoon.
‘If you can assist, please call 101 quoting 44220186268. Alternatively, you can call Crimestoppers anonymously on 0800 555 111.’
A community fundraising effort has been launched to support mum-of-five Dana Myers who says she has a lost ‘everything’ she owned after the blaze guttered her flat.
Hampshire and Isle of Wight Fire and Rescue Service dispatched over 50 firefighters from across Hampshire to tackle the blaze, including crews from Southsea, Cosham, Portchester, Havant, Fareham, Hayling Island, and Waterlooville.