The teenagers used lighters, aerosol canisters and pieces of wood to start two blazes in and just outside Fraser Range, the ex-naval training centre in Eastney.
Firefighters were made aware of the incident when nearby fisherman alerted them just after 3.30pm. Upon their arrival at the scene the teenagers hurled stones at the crew.
Southsea Fire Station crew manager Rob Gargaro said: ‘It was a very intimidating situation.
‘The youths harassed the eight of us that attended and after we went inside the building to extinguish the one fire they’d set, they’d started another one outside by the time we came out.
‘We requested the presence of police and only as they arrived did the teenagers disperse.
‘They think it’s okay to go in there and play, but they’re putting themselves in a very dangerous situation.’
The crew said that not only do the youths put themselves at risk, they stop the firefighters attending more serious incidents.
Mr Gargaro added: ‘The time and resources we waste on incidents like this stop us from attending situations where lives are in danger, and one day it could be the families of those teenagers that need help.
‘The danger of playing in a derelict building is that parts of them could fall down and land on whoever is inside, injuring or even killing them. There’s lots of glass on the floor in this one and there’s a high risk of cuts.
‘We want people to be aware of the dangers of this kind of behaviour, it’s not acceptable and we’d like people not to enter derelict buildings.’