Crime scenes officer dressed in white suits and protective equipment are scouring the charred remains at the former Playlands amusements arcade opposite McDonald's in Commercial Road.
It comes after firefighters said they tackled a ‘raging inferno’ at the building at around 3.30am this morning. No-one was injured.
Detectives from Hampshire police are in the street.
A Hampshire police spokeswoman said: ‘We are investigating the exact circumstances surrounding the cause of the fire, which is currently being treated as arson.
‘Officers will be in the area today, so if anyone has any concerns, please go and speak to them.’
Onlookers stopped to take in the devastation at the northern end of the city’s main shopping street.
It comes just two days after many stores reopened with strict social distancing in the coronavirus lockdown.
The Vapour Room, in the same City Buildings block as the devastated derelict amusements and right next door, suffered some soot damage in the back storage area.
Speaking to The News inside her store, owner Charisma Webster said she was called at 4am – and quickly got to the store.
‘My instant thought was panic, “is everyone alright?” I thought everyone would be gone at that time in the morning – you don't expect anyone to be here.
‘I got here, there were loads of police cars, fire trucks – I couldn't believe the damage – I prayed for my business.
‘It's not the best thing to happen considering I’ve finally managed to open after three months of lockdown.’
Charisma said she feared the company, which has been running in the spot for five years, might not make it ‘back again’ in lockdown.
‘And then something like that happens,’ she said. ‘I’m very thankful that it wasn’t (on fire) a couple of minutes more.
‘This is an electric cigarette shop – there are batteries, it's potentially a big fire hazard.
‘We've worked so hard to get this and to work so hard for this – it's too close for comfort.’
Charisma heaped praise on her team who have joined in the clean-up, and on firefighters who saved her firm from disaster.
But she said it should be a wake-up call to shoppers – the fire putting into sharp relief how lockdown has put independent stores on the edge.
Charisma feared the fire burning just minutes more could see her store ‘potentially no longer existing’.
She said: ‘It’s a real wake-up call – it’s extremely scary.’